The rules state that assets must be properly credited, but does not restrict where they may be from. Are there any limits or would, for example, a bootleg-esque heavily crushed 2 second sample of a commercial song be acceptable to include?

No limits on source as far as the jam goes, if something will cause a problem on-stream we'll usually just ask for a new version of the game to be uploaded with the problem content removed. Bitcrushed sample should be fine.