Solved this now but for anyone else out there (and in my google search results,
it looks like there are a few):
until yesterday I had no problems playing music on lenny, but after loading the
latest updates, which were much more numerous than usual, my songbird refused to
play anything, giving the error about autoaudiosink and alsasink both being
Plus the audio control panel on the desktop (the xfce plugin for me) had no
controls in it anymore except the mic.
After trying a few things, I just did the following: