Looks like there are a bunch of Famitracker modules (.ftm files) that are converted to MIDI but should actually be converted to NSF like any other NES song.
Oh, yes. That's because the Famitracker port I'm using[0] is a bit old and doesn't support the newer .ftm versions. So in case the player doesn't support a given song, there's a fallback to using FTMtoMID so you can still listen to it.
This was all done before I ported Furnace Tracker (that also supports the .ftm format) so I might just use Furnace for playing them. I have to check whether it can play the newer .ftm versions.
Also SID files play for a fixed amount of time, so long-ish ones cut during playtime. I know HVSC has a database that defines track length, not sure for the others though.
Yes, as I mentioned in another comment, the player has a time limit on songs to prevent them from looping forever. I'll include an option to let you modify this, so you can listen to them _almost_ forever.
This was all done before I ported Furnace Tracker (that also supports the .ftm format) so I might just use Furnace for playing them. I have to check whether it can play the newer .ftm versions.
[0] https://github.com/nukep/famitracker-cx