How do you expect it to know which volume setting you want to adjust?
Honestly, the most intuitive option for me would the audio controls should never touch the ringer, bury that behind a menu, rather than switching based on context.
You can always adjust the ringer (something people rarely do -- for quite times there's the "silent" mode anyway) from the UI, and always use the volume buttons for the system's (music, video etc) audio volume.