audio-play! returns the file id.

The file id is now a randomized number

scrbl/audio-placed-player.scrbl

@@ -72,7 +72,8 @@ The important command-level behaviour is:
  @item{@racket['init] installs the reply and event channels and moves the
        player into the initialized command loop.}
  @item{@racket['open] starts a decoder and a read worker.  If another worker is
-       still feeding audio, it is first interrupted and joined.}
+       still feeding audio, it is first interrupted and joined. Returns
+       a @tt{music id} for the given file.}
  @item{@racket['pause] only changes @racket[player-state].  The worker observes
        that state and applies @racket[ao-pause] to the output side.}
  @item{@racket['seek] clears the async output queue and seeks the decoder, but
@@ -96,7 +97,8 @@ an RPC-style command is sent on the reply channel installed by
  @item{@racket[(list 'init ch-out ch-evt)] installs @racket[ch-out] and
        @racket[ch-evt], and replies with @racket['(initialized)].}
  @item{@racket[(list 'open file)] opens @racket[file], starts the decoder
-       worker, and replies with @racket['(ok)].}
+       worker, and replies with @racket[(list ok music-id)], where
+       @tt{music-id} is the given music id to the file to play.}
  @item{@racket[(list 'pause paused?)] sets @racket[player-state] to
        @racket['paused] or @racket['playing] when the player is already active,
        and replies with @racket['(ok)].}