I can't work out how to change the volume of a stream. I can't find any docs for LOGG (but there appears to be no function for changing volume); I also can't find anything about changing volume in the libvorbis docs.
LOGG uses allegro's audio streams (I think, I hope, I'm guessing...), so from the manual entry for play_audio_stream, you can use the voice functions to change the volume.
If you want to adjust the pitch, volume, or panning of a stream once it is playing, you can use the regular voice_*() functions with stream->voice as a parameter.
Untested. I assume in the LOGG header you can find the voice parameter to pass to the functions.
I had never noticed that Allegro had its own audio streaming stuff - until I was stepping in the debugger and it stepped into that code. I added a function to return the voice for a stream.