I haven't used it, but wanted to share it, since someone mentioned he got his older Digidesign MBox work with it again, the Intel version costs a bit and the PPC-version is FREE 
http://www.macupdate.com/app/mac/11730/usb-audio-driver
There are also some MAudio USB drivers for PPC (from MAudio itself), I didn't know they did exist.
Edit: I thought I just add them (not sure, if complete)
- m-audio mobilePre http://www.macupdate.com/app/mac/13755/m-audio-usb-mobilepre
- m-audio Omnistudio http://www.macupdate.com/app/mac/13756/m-audio-omnistudio-usb
- m-audio USB Quattro http://www.macupdate.com/app/mac/13753/m-audio-usb-quattro
- m-audio audiophile http://www.macupdate.com/app/mac/13840/m-audio-audiophile-usb
...and while we're at it, some more audio drivers for Digidesign, Motu, Behringer http://download.cnet.com/mac/audio-drivers/?osNamesExact=Mac+OS+X+10.4+PPC,Mac+OS+X+10.5+PPC,Mac+OS+X+10.3.9,Mac+OS+X+10.2,Mac+OS+X+10.3,Mac+OS+X+10.1,Mac+OS+X+10.4
and "Classic" http://download.cnet.com/mac/audio-drivers/?osNamesExact=Mac+OS+Classic
I assume some of those came already with the equipment one bought or were included anyway...
http://www.macupdate.com/app/mac/11730/usb-audio-driver
There are also some MAudio USB drivers for PPC (from MAudio itself), I didn't know they did exist.
Edit: I thought I just add them (not sure, if complete)
- m-audio mobilePre http://www.macupdate.com/app/mac/13755/m-audio-usb-mobilepre
- m-audio Omnistudio http://www.macupdate.com/app/mac/13756/m-audio-omnistudio-usb
- m-audio USB Quattro http://www.macupdate.com/app/mac/13753/m-audio-usb-quattro
- m-audio audiophile http://www.macupdate.com/app/mac/13840/m-audio-audiophile-usb
...and while we're at it, some more audio drivers for Digidesign, Motu, Behringer http://download.cnet.com/mac/audio-drivers/?osNamesExact=Mac+OS+X+10.4+PPC,Mac+OS+X+10.5+PPC,Mac+OS+X+10.3.9,Mac+OS+X+10.2,Mac+OS+X+10.3,Mac+OS+X+10.1,Mac+OS+X+10.4
and "Classic" http://download.cnet.com/mac/audio-drivers/?osNamesExact=Mac+OS+Classic
I assume some of those came already with the equipment one bought or were included anyway...
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