We’ve added a couple of audio settings in beta 2C to help track this issue down.
In the sound options window (from the main menu, select ‘options’, and then the sound tab) change ‘use system reverb’ to ‘off’ and ‘Audio Fidelity’ to ‘Low Quality Audio’
Please try these settings, and let me know whether or not the problem still exists. If the issue is with the sound system getting bogged down under heavy cpu load, this should help.
As always, make sure your sound card drivers are up to date.