Basically, it's okay to upload music that was released for free elsewhere (by the author). Legality aside, that's pretty much how it's been done up till now.

If the music was released under a license that forbids redistribution, you should really choose another track to accompany your level. But it's pretty impractical for us to check all uploaded music's license so there's probably quite a few tracks on J2O that violate that rule.

Rules of thumb:

• Mods, like those that you can find on modarchive: yes
• Music that has been released for free and does not forbid you from (re-)using it: yes
• Game music from old games (like the stuff you can find on mirsoft.info): technically probably not allowed, but no one cares
• Game music from recent games (say post-2000), or commercial music (Madonna): no

It is always preferable to upload your level's music to J2O together with the level. External links break after some time more often than not, and we now allow large zip files to be uploaded.

So you have a choice: either use that one track that really fits best but is a song by Vanilla Ice so you can't really use it, and no one can hear your level's music now (if you just don't include it) or in a few month's time (when the external download link goes dead). Or just get a good track from modarchive or somewhere else and everyone wins.