Well for MIDI sequencing and/or MIDI combined with audio, you could have a look for Opcode Vision and Studio Vision which were released as free downloads several years back. I'm not sure where you would find them these days though.

Deck II is an excellent multi-track audio program for OS 9 and below, with fairly basic MIDI integration - though there is also a companion MIDI sequencer called Metro that is supposed to run very nicely alongside it.

There's a Digidesign Samplecell PCI on ebay somewhere at the moment, which is a hardware sample player on a card, with Mac OS control and editing software.

Do you own a mixer?