A very simple explanation t is so the volume on commercials is the same as the program. I am sure it has to do with the different sources audio is recorded in or what channels they are recorded on when they are originally programmed. I am not sure but that would make sense to me.
I have other TV's that have this on it. The only thing that made me think of it was because the hotel we were staying at, when a station would go between commercials and a tv program, the volume fluctuates. I remembered at home that I adjust ours to "ON" on the TV's because it prevents going from a loud commercial to a quiet TV program or vice versa.
I will bet since I am using a "TV" as the monitor for my MAME that when you hit the fire button or and explosion occurs, the "TV" doesn't know really what to do so it just fluctuates. I am sure with the video games because of all the various sounds that it simply confuses the "TV" as it was never intended for this specific purpose.
Furthermore, as I said in a previous comment, i am a registered user of Maximus Arcade Software and I was really hoping to cure this problem as it is flawless to this point. My daughter and friends play this all the time at my house and several have asked me to build them one.
I am attaching a photo, I am going to Vinyl wrap the cabinet, still need to install the buttons. Made the control panel out of HDPE and need to install the monitor. I bought the Marquee at bargain price of $9.99 online. Got the marquee light working in this cabinet and have everything else ready to go. Just been playing the MAME on a work bench using my XArcade Solo until I finish this.