I have an odd situation. I have a recently installed Kubuntu 14.04 that I've been using as a work environment. It's not uncommon for me to use pianobar or MPD to provide background music, but still use videos in my browser, or Hangouts at the same time.
I've been able to any and as many sources at once as I wanted until I tried installing Pithos (a GUI Pandora player). After doing so, I can no longer use MPD or pianobar concurrently with other sound sources. I uninstalled the packages that were installed with pithos as such:
sudo apt-get remove gstreamer0.10-plugins-bad libcdaudio1 libgstreamer-plugins-bad0.10-0 libslv2-9 pithos python-gst0.10 python-libxml2 python-xdg
And even tried completely purging alsa-base and pulseaudio then reinstalling them with the Kubuntu-desktop metapackage. Still nothing. I would like to regain factory audio functionality without reinstalling my OS, which is what I usually have to resort to when Pulseaudio misbehaves.
It would appear that Pulseaudio is losing exclusive control of the sound device, perhaps? Not even the audio 'pop' sounds play when I adjust the volume while MPD/pianobar is running. Below are some of the other questions I've referred to:
This seems to work somewhat:
rm -r ~/.config/pulse; pulseaudio -k
Starting Pianobar before starting a Pulse-compatible source still seems to lock the output, but starting it after works, as long as the Pulse source doesn't stop playing. MPD is still exclusive--either MPD or Pulse, but not both. I've also lost the volume-change sound completely.