Incredibly minor, but odd nonetheless.
Make sure you're using Windowed mode. Load a game of any choice, wait until some music plays, then left-click anywhere in the gray menu bar (e.g. File/Machine/Netplay/...) that doesn't contain a menu item (e.g. the empty areas of the menu bar).
The audio stops for a brief moment, then resumes.
Below items make no difference:
* Enabling "Run in background",
* Increasing process priority,
* Options->Sound, choosing "System" memory pool (vs. Hardware),
* Options->Sound, decreasing/increasing latency,
* Options->Timing, unchecking "Use high priority timer" (HPET)
Appears to be purely a UI bug of some kind. Probably a result of the "beauty" that is involved in handling every single user interaction possibility in a Win32 app. :-)
Also, right-clicking doesn't cause this to happen, only left. Although, unrelated, right-clicking while hovering over a menu item results in the outlined selection box disappearing -- equally as bizarre...