I'm trying to use a single headset for both listening to music and talking on the phone (via Google Voice). It's more difficult that it sounds! One big problem I found is that when talking on the phone, I have to jack-up the output volume (to close to 100%). But, at that setting, music playback is extremely loud---I prefer approximately 33%. Fortunately, there is a way to modify the volume of a particular application. Under Preferences->Sound->Applications, I found an "ALSA plug-in [ogg123]" control. I play music with ogg123. I can use this slider to control volume of music playback. By setting this slider to 33%, I can set the overall output volume to close to 100% and not have to switch volume back-and-forth when I go from listening to music to talking on the phone.