Comment 5 for bug 374059

In Lucid Lynx (at least with me), mpd plays music through pulseaudio correctly, but it appropriates of pulseaudio's output. So, while having mpd playing music, other apps cannot use pulseaudio output 'cause it's owned by mpd. And this was my workaround:

Adding mpd user to pulseaudio groups didn't work. So, the simplest way was replacing the mpd user from the /etc/mpd.conf file by my user (rcart). After restarting mpd, it will claim 'cause the user has not permissions on the /var/run/mpd/pid file, and this is solved by changing the pid file owner to the user (in this case rcart).

Those lines look like this:

/etc/mpd.conf:
       user "rcart"

/var/run/mpd/pid permissions:
       -rw-r--r-- 1 rcart root 4 2010-07-30 16:30 /var/run/mpd/pid

Hope that works for those who have the same problem =)