Even if a key witness is deaf, cannot speak, and has not learned sign language, she can still properly testify at a criminal trial.
Sure, a defendant's ability to cross-examine that witness is somewhat limited, since she's using somewhat ambiguous facial expressions, gestures, and lip movements to both understand and respond to questioning. So her answers -- and her ability to understand the questions posed to her -- may often be unclear. But you simply do the best you can; the Confrontation Clause does not preclude the jury's reliance upon her testimony.
That seems right to me.
I also found tangentially interesting an unpublished California Court of Appeal opinion that Judge Fletcher discussed in her opinion. That opinion, rendered in 1996, affirmed a conviction when a key witness -- who was left speechless and a quadrapalegic after being shot -- was permitted to testify by tapping once for "yes" and two times for "no".
Now, I'm not a super geek. At least compared to the thousands of people who are currenly flying into San Diego this week. But upon reading this, my mind instantly returned to a classic episode of Star Trek in which Captain Christopher Pike (the original commander of the Enterprise) testifies in almost precisely such a fashion, beeping (through a machine) once for yes and twice for no. (I'd be even more of a geek if I admitted that I also knew that there was a Futurama episode that mocks this episode, and in which Fry testifies in the same fashion, so I shall decline such an admission.)
Sometimes life imitates art.