Handling sound playback sounds like something that would be better suited for a Mediator & component pair. I like to treat my proxies as holders of data that extend a few specific methods for the data. Methods that return id3 information would be good to have in the proxy, but in terms of controlling what the volume level is, I would be putting that in the mediator that facilitates audio playback.
So rather than a PlaybackProxy, I would have your Mp3Proxy, or whatever you would choose to call it..and then a PlayerMediator that is responsible for plugging all of the proper stuff (the sound data, id3 tags, volume level) into your Playback component. If you were storing the volume that the user set in a past session and then bringing it back then it would make sense to keep that in the proxy because you will have to load/save it when you start/close the application. But when I get too many user settings I like to create an ApplicationStateProxy that is responsible for tracking all the things that the user has set that would need to be remembered. Then I have that proxy store it's data in a
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/net/SharedObject.html.
