I agree that a(nother) good audiology exam sounds like a good idea, but I wouldn't worry unduly. There does seem to be a personality type that combines a tendency not to do things until really good and ready and then learn them very fast, with being highly strung and easily overwhelmed in new situations; I have the feeling that we see it a lot around here, though maybe it's not correlated with being here!
FWIW my DS wasn't really talking at all at 22 months (nor signing like your DD) but his speech took off dramatically after that - I've told the story before of being unable to tick the "uses 5 words regularly" box at his 22 month check, and yet he was using 5 word sentences by 26 months. Seems there are quite a few stories like that. He's the kind of child that makes you see why people describe the autistic spectrum as one that goes all the way to normality - he's a geek from a family of geeks with plenty of oversensitivities, and when he was younger I did sometimes wonder whether he might be diagnosable - more in a "might someone misdiagnose him with an ASD" way than a "does he have an ASD" way - but now that he is 8, that seems a bit silly.
