If you are using an actual telephony headset (usually USB connected) then run the "Set Up Audio Video Wizard" and on the Sounds page make sure your PC Speaker is selected and not the headset/speakerphone device.
But if you simply have a standard set of headphones connected to the audio jack then you can't direct the program sounds in OC (such as IM alert or in-coming call ringing) to the standard PC speaker because the headphones are now the default PC audio device.Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS