The wavTrigger from robertsonics is a much better choice than the mp3Trigger for custom pin sounds. Same price, more functionality. I am unfamiliar with the offerings of adafruit or raspPi, so I cannot comment on what they offer, though other programmer friends say they are good.
But as far as the wavTrigger: if all you want to do is add bg music it is quite simple enough to use one of the physical contact inputs on the WT and wire it to the out hole switch. The programming on the WT has many options including a "STOP All Tracks" function.

I'm currently working on a setup using the WT and an Arduino, with the Arduino using the serial port to communicate with the WT and acting as an "interpreter" between the WT and the original commands the soundboard would have received from the MPU. ..a little more involved on the programming side than the very simple WT utility is. When I get all the kinks worked out, I will be more than happy to share my Arduino sketch. But as I mentioned, the WT utility program is very user-friendly and easy to understand if you are wanting to find a way to use just the WT alone.

checked out www.robertsonics.com I assume?

From the robertsonics website: "Trigger inputs support passive (contact closure) or active (3.3V / 5.0V logic) as well as inverted modes on an individual basis."

Though, in all fairness and honesty, I must offer up that I have not actually tried this method of direct connection to an existing switch. I do have the WT and am currently working on it but I will take some time aside to experiment and see if this method does or does not work. I'll report back with my findings, probably Wed evening.

After some quick experimenting with all of the various "trigger type" options with the WT, I can say in full confidence.. I am incorrect, toyotaboy has it right: You cannot use a switch that is wired into the switch matrix, it does give false triggers. It is not as simple as just wiring it in.
BUT!! it CAN be as simple as replacing the existing single switch with a dual switch. Not a dual contact switch, but a dual switch with two sets of blades that can be triggered by one actuator. One set gets the wires that the original switch had, the other set goes to the WT.

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True, $8-10 a switch can be pricy when trying to do an entire game.

I'd only make this suggestion for an application such as Chosen_S was asking about where he only wants to be able to start and stop background music.