On my laptop running Ubuntu (Intrepid Ibex), I've had difficulties getting audio to work simultaneously in both Firefox (Flash, to be exact) and my music players. I can play music in RhythmBox or Movie Player, but if Flash, running in Firefox, manages to grab audio, then the music players can't get it back.
The following steps from here fixed the problem. (As usual, the most time-consuming part was identifying exactly what the problem was. Fixing took a few seconds.)
$ echo "default_driver=pulse" >~/.libao
$ rm -r ~/.pulse ~/.asoundrc*
$ sudo apt-get update && sudo apt-get dist-upgrade
$ sudo apt-get remove libflashsupport
$ sudo apt-get install libasound2-plugins padevchooser libao-pulse libsdl1.2debian-pulseaudio
The instructions on the webpage were more detailed, described the purpose of each step, and included other things to check along the way, but in the end it came down to the above steps for me.