I haven't quite gone as far as to use a random internal music. Instead, what I've done is made it so that if it can't find the music outright specified by the level, it will treat it as if the level didn't specify a custom music (ie: use the normal rotation). If no rotation is specified for the pack, or if that fails too, it plays no music (without any popup message).
If a pack somehow manages to fail to the point where no music actually happens, then that is a result of either a "music pack is required" setup (possible but not overly recommended in the first place), or a design error by the creator that should've been picked up both by the issue checker
and during testing (if any).
That recommendation is nice, but the big problem remains how to get such low-quality versions. Programs like Audacity cannot export to .it (at least not that I know of) and not everything is already available as an .it.
Lower-quality and/or shorter OGGs is also a possibility (this will generate a warning but will not in any way fail to build - I'm not sure if it's easily possible to check the quality of them and only show the warning if they're high-quality - perhaps checking the file size is a better, albeit not-as-accurate, possibility); as is simply using other tracks - all that matters is that the
names match, NeoLemmix couldn't care less (or indeed, even know) that they're not actually different versions of the same piece.
I don't know that I want to go further than this, partly becaue this is something that content creators should be addressing rather than expecting the engine to fill in the gaps, and partly because, in all honesty, it would be a hassle to implement for very little gain.