Do you recall the System requirements of ProTools Free? I thought that was an OS 8.5/9.x only, but I guess the 5400 will run those. The SE/30 certainly won't easily and even so only outputs mono via Apple's sound manger.
I thought clevelander was looking for "music" software, like MOTU's Performer, or Cakewalk MIDI applications, not sound editing software. However, I forgot about ProTools free and for the 5400, is likely a the best option for pure sound editing software. I was using it almost exclusively on my PowerBook G3 before OS X, which I prefer even over ProTools LE which REQUIRES an external box even for simple two track stereo. In fact I am going to set up my old PowerBook as soon as I transition completely from it to my MacBook, back to OS 9.2 and make it a dedicated ProTools Free machine. Thanks for reminding me, it will hasten my otherwise procrastinated project.