I tried a USB audio dongle (Turtle Beach Audio Advantage Micro) and mpd with my plug, but for some reason, only the right audio channel plays. It works fine on Windows with both headphones and speakers, so it's not a hardware issue. I've read over the PlugWiki USB Audio page (http://www.openplug.org/plugwiki/index.php/USB_Audio
) and searched on Google a bunch, but I can't figure out why only one channel would play. Any suggestions?
(One problem I did figure out was the audio quality. For some reason, the default mpd config file plays one-channel audio but with terrible crackling sounds. Uncommenting the alsa audio_output lines in /etc/mpd.conf as described in USB_Audio got rid of the crackling and reduced CPU load.)