A few of the live songs on Ummagumma were taken from the first 2 Floyd albums such as Astronomy Domine and, I am not sure, but it sounds like Syd singing on that particular track.
http://pinkfloyd-co.com/disco/umma/umma_album.html
Yup, sounds like him, but for whatever reason no credit in the disc's literature.
I had always hoped that he'd get his, um, stuff together and write/sing/play more. Sigh- no chance of that now.
maybe they'll find a chest with a hundred reels of 2 inch Ampex with unheard-of tunes on it...