I don't know if I have a broken copy (only 20 KB) but whenever I try to run it in DevKitPro (MSYS2) it doesn't output any message, and there is no sseq file. And yes I am using the right syntax (./midi2sseq.exe tmp.mid overworld_minor.sseq)

Download a different midi2sseq copy? and run it from cmd? It works fine for me. Maybe you arent specifying the output directory correctly? Usually, nsmb hacking software occasionally outputs files to a hidden directory. occasionally