Cassette Craze

It started with logs and diaries, as an easy way for characters to monologue information that was too time consuming to explain, or just happened off screen. Then came the Cassette Craze, when characters would carry around portable recorders to record the adventures of guys who traveled a lot because writing is not cool, and diaries are for sissies. This would evolve into the Captain's Log later, but that's Exty Years From Now, and will probably never happen anyway.

Of course, this is a Justified Trope when the speaker is a writer, teacher, journalist, researcher, doctor, or scientist, because it makes for better accuracy when there's a voice behind the words, and it's really used by said professionals in the field. It makes for more detailed information as the person works and better recordkeeping.

It also makes Apocalyptic Logs that end with the death of the speaker a lot more plausible. Who writes while they're being devoured? It's the in thing to beg for mercy and scream, and you just can't do that on paper.

Anime and Manga

 * In the Witchblade anime, Sohou Reina aka 'Lady' is obsessed with self-exploration (possibly trying to figure out how to be normal human despite her abnormal upbringing), so she keeps a recorder and dictates her impressions whenever she has any.
 * This is used very ominously at the end of Higurashi no Naku Koro ni's Tatarigoroshi-hen, in which a tape with unknown origins (but is believed and implied to be an interview of done by a journalist) is discovered and essentially ends with  going on a Laughing Mad tirade. The DS novel even shows a picture of him completely nuts, although how you can see that on an audio tape isn't really ever addressed.

Film

 * In The Movie 1408, Mike Enslin keeps records on tape recorder, a little more justified since he writes books about hotels at which he stays, and it's all taking notes.
 * In the story the movie is based on, he found that when camping out in graveyards, the tape recorder is easier to use than paper.
 * Flatliners: Randy Steckle (Oliver Platt) is constantly talking into his tape recorder, making notes about the experiments and situation and taping what the other characters are saying.
 * The Lost in Space movie has Dr. Robinson using his recorder gadget to remind himself of all sorts of things, like taping an apology for missing his son's Science Fair.
 * Tom Ripley in Wim Wenders' Der Amerikanische Freund (The American Friend) tapes himself. Later he gives the dictaphone to his German stooge as an impromptu gift.
 * Dr. Conrad Zimsky (Stanley Tucci) in The Core has a tendency to constantly and pretensiously narrate the journey for his upcoming biography, much to the annoyance of the other crew members. Subverted when
 * In Primer Aaron constantly records everything people are saying so he can play it back.
 * Norm MacDonald's character in Dirty Work is constantly making notes-to-self on his dictaphone, which becomes an important plot device later when he accidentally leaves it on. Notably, the "note-to-self" thing was already an element of Norm Macdonald's comedy before the movie came out.
 * Unbuilt in Horror of Dracula: Van Helsing records notes on a nifty Edison wax-cylinder phonograph (probably a Shout-Out to Seward's phonograph diary in the novel). Alas, it only plays out as a joke when a servant wonders who the good doctor is talking to, and is somewhat taken aback when Van Helsing says he is talking to himself.

Literature

 * In The Handmaid's Tale, the stories told by Offred are revealed to have been discovered recorded on cassettes at the end of the book.
 * Dracula: Dr. Seward uses a phonograph to record his observations.

Live Action TV

 * In the Famous Ancestor episode of Star Trek: Voyager, Shannon O'Donnell, Janeway's surprisingly identical ancestor, uses a tape player to record logs of her travels.
 * In Farscape, John Crichton uses a tape recorder to record messages to his dad. You think he would have picked up something better at an alien pawn shop, but nope.
 * Of course, he'd be certain that if the tapes ever made it back to Earth that people would be able to play them.
 * The real weird part is that he never runs out of tape, even though the original mission was only supposed to last a few hours.
 * Parodied on Scrubs: J.D. and Turk claim to carry a dictaphone around to take patient notes, but they actually use it to tape their singing when no-one's about.
 * Alex in Ashes to Ashes records her experiences on a tape recorder, addressing her daughter in them.
 * Diane, I've noticed that Agent Dale Cooper in Twin Peaks constantly records his observations on his handheld cassette recorder, including personal opinions having nothing to do with his job.
 * In the Pilot Movie The Invisible Woman, Bob Denver's character used one of these devices.
 * In The X-Files, Scully practically abuses her recorder in order to tell the audience useful tidbits about the case, usually when she is performing an autopsy.
 * Many agents are shown to use cassettes to record their cases. In "Lazarus", Mulder listens to the cassettes of Jack Willis as he tracks down a Bonnie and Clyde couple. Scotland Yard agent Phoebe Green also used a cassette to relay the basics of the case she was working on and needed help with (upon it's finding, Mulder quips that "10 to 1 you can't dance to it."). She also leaves one at the end of that episode, but it is never listened to. Even Mulder is seen to use a tape recorder; during "Little Green Men", he uses one to record his findings in the hopes that Scully would find them. Likewise, during "En Ami", Scully sends Mulder cassette tapes to alert him as to where she has gone with CSM.
 * Confessions of suspects are also usually put on cassettes; Mulder listens to Duane Barry's confession repeatedly during Scully's abduction in season two.

Newspaper Comics

 * Luann's titular character uses one of these as her diary.

Video Games

 * In Bioshock, the citizens of Rapture kept their diaries, notes and messages to each other on personal voice recorders, all of which you find lying about the city. This allows you to listen to their Apocalyptic Logs while walking around.
 * As did its predecessors, System Shock 1 & 2.
 * Similarly used in the PC game Iji. Logs and messages from both invading alien races can be found all over the place, often with reams of subplot, and occasionally with hints about unlocking special skills.
 * Doom 3 also used this. As in System Shock, they would sometimes contain door access codes and computer passwords.
 * Dead Space also sprinkles audiologs around like it is candy, with characters telling about how everything went bad. In one extreme example, a man record his monologue while he is sawing of his legs.
 * A lot of those are justified, as they are actually Black Box recordings from RI Gs, backups from security recordings, or sessions of somewhat flimsy medical experiments. Or, of course, people simply leaving a warning in a bit less blunt of a way than painting it on the wall in their own blood. It's the occasional, random, everyday stuff meant to make you empathize with people who died that you'll never see that makes no sense.
 * Done in Tomb Raider: Underworld, to allow Lara to give hints without having to use her Voice with an Internet Connection, which annoyed a lot of people in Tomb Raider: Legend.
 * Fallout 3 does this fairly often.

Web Original

 * This is pretty much the basis of Marble Hornets. Justified early on by the fact that most of the characters are film students, and later since the resident Humanoid Abomination shows up on tape even when people can't see/remember it, so taping yourself can be a damn good idea.

Western Animation

 * Bart Simpson uses one.
 * In Metalocalypse, Nathan Explosion carries a digital recording device at all times in the event that he has an idea for a song out of nowhere.
 * In the One Hundred and One Dalmatians animated series, Cruella de Vil is often seen speaking into one: "Memo to myself:..."
 * AndrAIa blatantly Expospeaks into a cassette recorder about her and Matrix's journey through the games. Including covertly expressing concerns about Matrix's morale, even though he's standing right next to her. And then the cassette recorder is never seen again.
 * In The Venture Bros episode "Ghosts of the Sargasso," Doctor Venture's pretentious narration gives way to panic and desperation as his underwater exploration doesn't go as planned.
 * Parodied in King of the Hill, Crazy Survivalist Dale Gribble speaks into a tape recorder when he believes he's been fatally infected with rabies and proceeds to flee into the wilderness in delirium. He lists off various items to bring with him next time he flees into the wilderness, including batteries for the tape recorder. He then proceeds to toss the empty tape recorder into the forest, and finishes his diary by speaking into a pinecone about how he'll also need to bring a new tape recorder.