Ladydramaturg, Fred could sing but his voice was very thin. Onstage, he was often overwhelmed by the orchestra but when he danced, nobody minded. I have found articles in British newspapers mentioning this. Stage Magazine gave Gay Divorce a scathing review because nobody in the cast could sing (a common note in most of the Gotham reviews.) When Fred was in a recording studio, six inches from a microphone, THEN he could sing because he didn't have to force that reedy voice to fill a hall (and remember I ADORE him, this isn't news). Nobody could deliver a song like Fred Astaire. But filling a theater was not his forte.
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699