Is this during a game, from within the FE, or any time at all?
My first guess is that this is occurring when MAME is running, and you have somehow mapped a key to it's volume control.
Check your MAME settings and/or key output from encoder.
In Mame, hit tab, pick Input(general), then Other Settings (depending on mame version, wording might be a bit different). Volume should be found in the list there.
For your keyboard encoder, there are different ways to test output (check wiki for software to show key presses).