Comment 17 for bug 1325752

This is not abug (even I found it a bit annoying).
Mixx stops PulseAudio to get full control of sound. The only solutoin I found to make all sound at the same time is get all sound go through Jack.
Can't remember where I found how to do it but you can start:

 http://docs.fedoraproject.org/en-US/Fedora/16/html/Musicians_Guide/sect-Musicians_Guide-Integrating_PulseAudio_with_JACK.html

 https://wiki.archlinux.org/index.php/PulseAudio/Examples#PulseAudio_through_JACK