Not free, but Stardock's Acoustic Bridge might help:
Acoustic Bridge gives you the power to redirect audio from one PC to the speakers of another PC. This enables you to send the sound from a PC with your music collection to any other PC on your network. You could even visit friends and redirect your PC audio output to their PC and speakers. Use the best speakers available wherever you are.
Centralize the sound output from multiple PCs to a single PC. This enables you to transfer the audio output from any application, such as chat, e-mail, stock price alerts, or video/media players, from your desktop to your laptop.
Redirecting your PC audio output is as simple as clicking the Acoustic Bridge icon to toggle sound between your local PC and another PC. Acoustic Bridge will send or receive sound from any PC on the network that has Acoustic Bridge installed.