I can relate to your situation, without the autistic daughter part. My husband is also a musician, not his daily profession, and I'm often the only one who hears his work. For the most part, he plays his guitar and sings (things he has written) in the living room while I do other things like reading or knitting. As he is always tweaking his latest songs, what he plays can be rather repetitive, especially if he is having some trouble with lyrics, and I have a tendency to tune out after a while. I do genuinely like most of his songs, but when he puts me on the spot to tell him how I feel about a certain one, I sometimes struggle to be complimentary enough. Plus it seems like there is never a final version. And then he will say something later like "oh yeah, I forgot you don't like this song," even if that's not the case, because I wasn't enthusiastic enough about it before. I have been making a bigger effort lately by saying things like "I have always liked this song" or pointing out a passage that means something extra or that I noticed he changed. I also remind him that I like some of his older music that I haven't heard in a while. Sometimes he just needs to be reassured that I am listening. I do recall getting pretty misty about one song that was more personal when listening to a recording on headphones. That meant a lot to him.

If your husband has written something so personal as a special song about your daughter, I think that requires extra attention from you. But make sure he understands that you need a quiet time to take it all in and can give it your full attention, not when there is a lot going on at once. Maybe ask to read the lyrics so you can be more involved, and use them as conversation points -surely you can find some common ideas there if it's about your daughter. Point out parts you like, and keep anything negative to yourself, or that may be the only part he hears.