Touch (manga)/WMG

Jake isn't mute

 * His opening and closing monologues clearly demonstrate a high level of intelligence and he speaks in them. I posit that he actually is capable of speaking, but doesn't do so because he hasn't come up with any way to verbally communicate what he sees. (Or perhaps is simply aware that if he were to tell people "I can predict the future", they would think he was insane. Given his obssession with the numbers and patterns, it's likely that he doesn't consider anything else worth communicating about, so he simply doesn't talk.
 * This is made more likely in the season 1 finale:

The Japanese girls will eventually become part of a much larger Story Arc.

 * They've appeared in three consecutive episodes so far. No way is that a coincidence.
 * It's possible that they, along with other throw-away lines, references, and events through the show will figure into some grand season finale.

Teller is crazy

 * In school, Teller was really into math and sci-fi. Rather than become a writer or a mathematician, he slowly descended into obsessive madness. The series is his hallucination.
 * Probably Jossed, considering

The numbers are the will of a superhuman entity

 * Amelia was its first victim, and now . Its influence on a mortal manifests itself as brain damage - hence why Teller's enlightenment was treated as an aneurysm, and Jake has an Ambiguous Disorder in-universe.