You can do that for yourself, for your personal collection - absolutely, no one's stopping you, and why would they...
The key word here is "redistributed" - if you're hosting/mirroring portions of the collection for *others*, yeah, we want the files to be official & unmodified. If you're remixing them, that's no longer redistribution of the original audio, it's a different ballgame entirely. Conversion to OGG wouldn't make much sense (lossy to lossy?) - @Liontamerhas done work on silence trimming so if you find mixes w/ 10+ seconds of true silence (no signal), let us know here. The bigger no-no that we're especially concerned with would be modifying tagging/attribution, FYI.