Force volume buttons to always change Media Volume
There is no setting for this as I'm sure you know. The only way l see this working is if you potentially write an app to capture all volume button changes and then act on the changes from within the app.

You could potentially use registerMediaButtonEventReceiver to register your app as a BroadcastReceiver for all media button calls. I say potentially because I'm not 100% sure if this BroadcastReceiver will handle these Intents since I'm purely looking at documentation and haven't actually tried this myself.