problem with sound in Call of Duty 2

I bought Call of Duty 2 while my grandfather was buying a new iMac. We brought it home, set it up, and CoD 2 worked fantastically on his computer. I brought it home to my older iMac G5, and tried to set it up. The graphics are fine, the gameplay is there, but the only audio that plays is the background music and environmental sounds (wind, distant gunfire, etc.). No gameplay sounds (gunfire, voices, footsteps, etc.) can be heard. I read up on this in the forums, where people had solved the same problem by going to Audio MIDI Support in Applications/Utilities and changing audio output to 44,1000 Hz 2ch. I tried doing the same thing, but the audio output pane in the MIDI support app only said "output not supported". How can I fix this? Please help!

iMac G5 2 GHz, 512 MB RAM Mac OS X (10.4.9)