audio-play! returns the file id.

The file id is now a randomized number

scrbl/audio-player.scrbl

@@ -76,10 +76,12 @@ is invalidated and @racket[audio-play?] returns @racket[#f].}
 
 @defproc[(audio-play! [player audio-play?]
                       [audio-file path-string?])
-         symbol?]{
+         number?]{
 Starts playback of @racket[audio-file].  The file is opened by the worker side,
 a decoder is selected, and audio feeding begins asynchronously.  In normal use
-the return value is @racket['ok].
+the return value is @racket[music-id], where @tt{music-id} is the
+given @tt{id}, a @tt{number? >= 1} to the file to play, which will be used when reporting in
+callbacks about e.g. state.
 
 Calling @racket[audio-play!] while another file is still active replaces the
 current stream.  The worker side interrupts the old decoder, clears the output