Considering that I watch movies and anime on my old 550mhz G4 Cube often, I periodically have with files encoded using multichannel AC3. And while I could just watch and listen in stereo, it'd be nicer if I could get surround sound on my Mac without having to buy a expensive sound card.
On a whim, I did some google searches and found that OSX 10.4's CoreAudio features the ability to aggregate sound devices. I thought this might be able to get me the equivalent of a surround sound device by just pairing up normal sound cards.
I attached an iMic to my G4 Cube along with the normal Cube USB speakers, went to /Applications/Utilities/Audio MIDI Setup.app, and made myself an aggregate device with the two USB sound sources. The speaker setup was set to quadrophonic. The iMic was connected to earphones for a test, and later a set of speakers.
After the setup, I loaded up a file with a AC3 sound track in VLC and hit play. A problem with an aggregate device is that there are no volume controls. You have to select each of them in the Sound Preferences and set their volume independently. But, in this situation, discovering that was still quite useful. I tried alternating muting and turning up the volume on the two pairs of channels and noticed that the tracks are indeed different.
I guess you could say this looks like a success. However, I probably won't be continually using it since USB audio sucks out too much CPU power. And because I'm using a G4 Cube, both my audio devices are USB.
Update 20060417:
For mplayer (cuz it sometimes plays better than VLC) to use this, here's what I put in the config file:
channels=4
softvol-max=400
af=channels=4:4:0:2:2:0:1:3:3:1