When an error occurs in the TortoiseCVS client, it plays a “breaking glass” sound that can be pretty annoying. To avoid this, rename or delete C:Program FilesTortoiseCVSTortoiseCVSError.wav

Note that the error sound plays even if uses Preferences to set Progress Messages to “Really Quiet” (although I suspect this preference controls the verbosity of text feedback, not audio — a poor labelling choice).