Go to the volume mixer in Windows 10 and expand it so that you can see the volumes for all applications. Then just turn off the one that's under Electrum. It really should not be playing sound anyway - I haven't found anything in the source code (so far) that would suggest that is happening, so perhaps it is part of the Windows Sounds that also has to be muted.
You're correct that it's not part of Electrum but my best bet is it's the sound that I've mentioned above.It's part of Window's alert when the user is clicking outside of the active window which wont be disabled by muting Electrum's sound in Volume Mixer.
I'm guessing that his bot is double-clicking or mis-clicking somehow.
As an addition to my post, it can be muted by setting the "Default Beep" sound to "none" in System Sounds.
Shortcut is in "Search->type and click: change system sounds->Scroll-down and click: Default Beep"; below, replace the default "Windows Background.wav" into "none".
