dd1 was/is another extremely musical ASDer. she sang before she could talk, and hummed when she couldn't work out the words/articulate them. It used to drive me bonkers trying to work out what new tune she had picked up at nursery (I know the obvious ones, but she came home with a real variety!) so that I could learn it too, and try to help her with the words.
for a long while, when she was 3/4ish, we went with it. it was one of the very few things she wanted to do, and at least she had an interest in something. we used it to work on other skills - eg, we borrowed a tape machine with a pressure switch from Portage, and worked on her cognitive skills - push the plate and the music plays, walk away and it doesn't. this also helped me out as I had not long had dd2, and couldn't be at her beck and call so to have her accessing her music independently was a bonus.
dd1 used to sing/talk using only the 'dar' syllable too - intonation and tune told us all we needed to know for quite some time!
we worked on singing (or humming, if I couldn't work out the words!) together, or singing alternate lines. I used her current obsession song wise to try to interest her in other areas - eg painting stars while singing "twinkle twinkle', or finding bells to ring while singing Frere Jacques. when she showed that she was trying to articulate the words, we tried to work on finessing her pronunciation (and looked up associated oral exercises to work on each syllable. to be blunt, we were lucky in this - dd1 did most of the owrk herself, using mirrors/windows, and she/we were lucky it paid off)
agree with Saintly that floortime/intensive interaction type exercises helped us a lot (we were instinctively playing this way with dd1), so again, we used a toy star (for eg) to play 'put the star on the...' - either furniture, coloured spots on the floor, body parts/people (eg 'on daddy's head', 'on mummy's arm' etc).
dd1 also loved posting, so we had a stack of laminated cards, and a toy postbox (the elc one is really quite sturdy!), and she had to point out the right coloured star, or try to say the colour, or say the number of stars, before she got to post it.
obviously the later activities came once she would entertain the notion of doing something other than singing - the singing/storybook only phase lasted a loooooooong time.
I did say a while back I would try to look out some progressional videos of dd1 learning speech/perfecting her pronunciation. I will do that now life has calmed down a bit with ds, and will link you when I have done so.