Um...what? Since when are MP3s a storage medium?
More like a specification for how data is ordered.
As a means for explaining data sizes to people, it is a useful construct.
I used to use the Bible. The KJV in ASCII is about 5 meg. Take the total size and divide by 5M, and people could actually comprehend approximately how much space that actually is. Works about the same for mp3s. I think most companies estimate about 5M per mp3 when they say a player will hold "10,000 songs" or some such.