BTW, I wouldn't mind trying to fix this myself if you can provide pointers to where in the codebase I should start looking.

Take a look here: https://github.com/solvespace/solvespace/blob/master/src/platform/guiwin.cpp#L1051

EditorWindowProc() handles WM_KEYDOWN, but Windows also sends a WM_CHAR message with scancode == 0x1c (enter key) and once that gets into DispatchMessage() the unhandled sound plays.

Check out sherief@0f6e47c - I'm not sure whether that's a fix or a hack, is it safe to consume WM_CHAR into the void here?

I took a quick look--seems that this is the correct way to do it, conceptually speaking. See e.g. https://stackoverflow.com/questions/3588526/how-to-turn-off-beeping-when-pressing-enter-on-a-single-line-edit-control-under

Actually, I think that if you change WM_KEYDOWN to WM_CHAR here, you'll get the same effect in a slightly nicer way. sherief@0f6e47c#diff-99db0ce0039e3ba6b61036af47ff544fR1062

OK, probably not gonna go through and read the CLA right now but I submitted a pull request and it's a trivial change that I just feel like throwing in the public domain - feel free to accept, reject, or re-implement it. Thanks for all the help!