However, most media type of files (such as mp3, avi, mp4, jpg…) are already in compressed format.
Using file compressing utilities are generally wasting the effort.
Since the hard disk storage price is so affordable, compressing or converting the audio to squeeze 15-20% makes no sense.

In case you still want more space to trade off the convenience you already enjoy, here is a comparison of different audio format in turn of performance and compression ratio.
http://wiki.hydrogenaudio.org/index.php?…

Summary:
1. 7zip, winrar, etc won’t make any significant different on media files
2. Converting a media file is the most popular way to minimize a media file
3. Decreasing the quality of the media file can also minimize a media file