Earcandy is a small applet for Ubuntu (and I believe other distributions using Pulseaudio) that can be used to automatically control and prioritize your sound based on rules you select. I can be listening to music on Rhythmbox, and when I play a video in my browser, the music shuts off until the video finishes. If I get a Skype call, all other sound will stop, without me having to do anything.
The defaults needed a little tweaking on my system--Earcandy takes over the volume control for individual applications, and the default for Rhythmbox was at 100%, giving distortion. Adjustment has to be done in Earcandy rather than the app itself--The app control will move, but immediately resets to the Earcandy setting. You can select whether or not to fade in gradually, and the fade duration--but the fastest fade setting is still too slow. Two videos playing at the same time in Chrome both played.
Finally some justification for Pulseaudio. Handy, but not worth the hassle caused by Pulseaudio.
Update: Not stable enough on my system--I've had it hunt for volume, or give pops and ticks. Not awful, but I quit using it.