Arnold's is just a file player though, right? If you want to sequence (write/arrange) your own songs, you need a fullblown sequencer. There's a shareware one called Timidity which IIRC runs on 68ks, 040s at least. Otherwise have a look for an old copy of Logic or Cubase. You can get ones that will run all the way back to the 128k.

You're aware that "MIDI software" won't play any sounds on its own right? MIDI just being a serial control language; it sends signals to a synth or sampler (which can also be software in the same box, or external hardware). Apple's QuickTime Musical Instruments are an example.

What Mac/s are you looking at for this?