Coming in on this conversation late . . .
Definitely get hearing checked out, but a couple of reassurances are 1) I recall reading in multiple places that children who learn two languages tend to reach verbal milestones slightly later for a while. Later, they end up having stronger verbal skills than their monolingual peers, but in the beginning they start out more slowly. ASL is a second language. Sounds to me like your daughter has quite good language acquisition in one of her two languages (150 words at 15 months is waaay ahead of the curve), which suggests the brain is working just fine.
2) Case in point: A friend of mine taught her daughter signs as a baby, and she picked up lots of them -- and didn't speak much till quite late. Her mother believes she *understood* English but simply had no reason to use it because she could communicate so well in ASL.
Hope you'll let us know how it works out!
Jenny
