Smpeg and 'listening' to audio

Ok, I’ve got an MPEG playing (say, an MP3 audio file). What would I need
to do to determine what’s actually going on, audio-wise?

Would acheiving this require some deep, API-level changes in the smpeg
library? Or is there some existing function or data structure I can look
at to find ot what data’s being sent to the audio device?