A few comments....
Just about a year ago, I finished work on a Boxcar Sound Module. Posted an explanation of how I did it on the forum. Told others I could make 'em up as a for-sale item, not as a business but as a hobbyist to hobbyist sale (like the items sold on our forum.)
see thread here: https://ogrforum.com/...57#72383129414353157
I explained that there were features that differed from the fixed-sound Lionel version in a number of ways....
Self powered so not track power loss issues...rechargeable with about 5 hour runtime between charges... sounds stored on micro SD card... multiple files stored on the same card... diy sounds.... noisy boxcar track included (like Lionel's only nicer according to those who have heard both)... repeating loop tracks... modular...install in almost any boxcar...controls (volume, tracks, repeat) are accessed thru boxcar door... easily changed to seasonal boxcars...decent speaker... etc. They are not triggered by motion, but are turned on and off so I guess if you start and stop a lot, that feature on the Lionel boxcars may be important.
Using a sound card, no bluetooth is involved.... so no pairing issues, no dropouts, no need for a phone or tablet,
Building these myself after lots of trials and errors, I introductory priced 'em at $75 plus shipping with sound card, complete module, rechargeable battery, and boxcar sound track. Not much interest. A few commented to me on the price (as too high) even though the less capable Lionel version costs even more and is built into the car and is not easily modified to a different line.
Between buying the required parts, building each one, and shipping it, I felt that I needed to cover my costs and time... but not a lot of response. Unlike many of GRJ's (very nice) projects, this was created by joining a bunch of available modular boards and wired together to fit in a small space.
Then the thread moved into a discussion on "how to do it for $10" or "can you use if for triggered stationary sound tracks?" ( completely different project!....see thread) So, with some frustration, I put the project aside.. sold a few, it is available, and works well... more info available by contacting me via email in my profile.
On the positive side, I have used it on our club's modular layout and people seem to appreciate the additional ongoing sounds on the train. The Thomas module is a favorite (of course). Those who have one like it!
More recent project... a remote control system that launches helicopters (using slightly rewired MTH helicopter cars) as they run down the track... always playing with new ideas... As I'm sure GRJ would confirm, "designing it" is most of the fun!