A while back I was writing a suggestion post and as part of it I found experimentally that the priority is ogg > wav > mp3 > it > mod > xm, which matches namida's, but is missing some formats I was unaware were supported (I never actually posted the suggestion, but I saved the post's text locally in case I wanted to come back to it later).
Note that NeoLemmix discards the extension when loading music, so if my_song.ogg exists, there is no way to make NeoLemmix play my_song.wav.
I
really don't think this is desirable behavior, because it means that music tracks can mask other music tracks without producing a "this file already exists" warning when inserting it into the music folder, but I digress.
Also, the files that come with NeoLemmix are
technically .mod files, but they've been renamed .it (apparently this was done for historical reasons with the file handling code, and then just never undone because it wasn't causing problems). This bothers me more than it probably should.
If you want to actually write .it files, it can be done through
OpenMPT. .it and .xm are quite similar as far as the feature set goes; .mod is rather limited and I wouldn't recommend it. .s3m is a bit unusual, as it can have samples and OPL-2 isntruments (I don't
think OPL-3 is supported, but I'm not certain). But if you're using OpenMPT now, it's probably best to use its own .mptm format if you don't need compatibility with other trackers simply because it has some nice features like VST support and quality of life stuff like a system to set the tempo that's actually intuitive.
Unless you want to work in DOS for some reason, there's not really any reason to use the original Impulse Tracker.
