Thanks!
No plugins, that's FamiStudio, free DAW to make music for NES (you can even upload it to actual NES game). It has original limitations and sounds the part. It's a bit wonky to use, but I never used it before the jam and was able to finish the song, so there's no steep learning curve.
There's also FamiTracker, but I didn't want to learn trackers for a new jam this time :D