Something changed with Audio/MIDI setup in 10.8 Mountain Lion, and it requires an extra step when configuring System sound playback.
First, see section 2.8 Configuring the StudioLive For Core Audio (Mac Only), pg. 15 in the StudioLive Software Library Reference Manual:
http://support.presonus.com/attachments/token/eh5t5cjiwhgv71d/?name...
The Stereo tab should be set to the channels you want to use for playback, per the manual instructions above.
The Multichannel tab should be set to Quadraphonic and set to channels 1, 2, 3, and 4
NOTE: If your issues persist, you can also do the following to reset your Audio options inside of Audio MIDI Setup.
1. Go to this folder.
Mac HD/Library/Preferences/Audio
2. Delete these two files located inside of this folder.
3. Restart your computer and attempt to route the audio through Audio MIDI Setup again. The problem should be resolved.