Losing sound in WinXP

I have Windows XP Pro installed in Parallels Workstation, which is running on a Windows XP Pro laptop.

When I start my Parallels session I have sound, but then after it runs for a couple days I'll eventually lose the audio until I re-start the Parallels session (ie. "reboot" the XP session in Parallels, but I don't have to shut down Parallels, I just reboot the virtual PC.)

Everything seems to be configured correctly, obviously, since it works initially. When it stops working there is absolutely nothing to indicate there is a problem, there is just no audio. The audio device in the device list doesn't show a problem, and all of the audio-related tools still work, it just has a lack of audio output. The audio on the laptop works, but it just doesn't work within the Parallels virtual PC.