Problems with multiple sound sources

I had problems with playing mp3 after a watching a youtube movie.
Found this solution:

Sound problems in Hardy? Multiple sources not working?

Hardy now uses Pulseaudio as the default sound backend in Gnome. Though this is a good development (esd wasn't maintained anymore, various apps use different sound backends), this also creates a lot of problems.

For me, the situation was as follows: I have an Intel HDA sound chip on my mainboard. Very low tech by today's standards, but otherwise works fine. My sound setup was the default as I had done a fresh install Hardy. This gave me a sound setup that either work for Pulseaudio (which is a drop-in for esd), so I had Rhythmbox working, or worked for Firefox (Flash), depending on which I started first. That sucks. Sometimes you want both at the same time, or just pause on to run the other, not closing it completely.

I think I have that working now pretty well. This is how it goes:

First, install some extras: libflashsupport [this I skipped having the latest flash], libasound2-plugins and libsdl1.2debian-pulseaudio. These are libs to support Pulseaudio for Adobe's Flash, for Alsa and for SDL. The SDL lib will replace the default Alsa backend for SDL.
This is what goes into your /etc/asound.conf or .asoundrc in your home folder:

pcm.pulse {
type pulse
}
ctl.pulse {
type pulse
}
pcm.!default {
type pulse
}
ctl.!default {
type pulse
}

Then run "asoundconf set-pulseaudio" to fix up your personal Alsa configuration to redirect to Pulseaudio.
source: http://www.wzzrd.com/2008/05/sound-problems-in-hardy-multiple.html