In Gnome2 I use Gnome PulseAudio applet 0.3 http://code.google.com/p/gnome-pulse-applet/ http://code.google.com/p/gnome-pulse-applet/wiki/Screenshots I gives me possibility to adjust volume level for two or more applications playing sound at the same time, for example when I listen to a song from one application and want to hear an alarm or sound notification or make a phonecall from other application. I know that this can be managed trough already existing sound voulme control applet but in preferences. I gnome-pulse-applet it is accessible just with one mouse click, which is very convenient. Please, make it possible for gnome-pulse-applet to work on xfce panel or better implement its feature in already used volume control applet.
I've also found Pulseaudio Mixer Applet https://launchpad.net/pama https://launchpad.net/~v-geronimos/+archive/ppa Just port it to Xfce or develop similar functionality, please. It is really very useful.
Check into xfce4-pulseaudio-plugin; xfce4-mixer is deprecated for 4.12.