instead of zipping it with a normal compression, zip it with no compression. As people said, compressing music (MP3) is useless as it is already compressed; you wouldn't gain a lot.
Now, instead of trying to compressing and decompressing, which takes a very long time, the program you use to unzip it with should only extract the files if you have choosen to not compress the files (in zip-terms: 'storing' instead of 'max compression' or whatever).
So to solve your problem, provided you do want to have it in an archive: unzip the whole archive again and zip it again, but this time do not choose a compression level. It doesn't matter what program you use (winzip, winrar, etc) and it even doesn't matter what compression specification you use (zip, rar, etc) as most specifications can all simply 'store' files...
But since you just 'store' the files, I don't see any reason why you want it in an archive. Even extracting 'stored' files takes a while. And then there is also the disadvantage that if your archive got corrupt, you might loose a lot of stuff.
So, in the end, best is just to make a backup in a folder on another hard disk or something.
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