Using stuff I had lying around, here's a video of one approach. Yes, there are i's to dot and t's to cross but with some DIY persistence I think it could be done for a modest out-of-pocket outlay of cash.
An AC-to-DC voltage converter takes AC voltage from the operating-car's power pickups and converts to DC - set to 3.8V. I removed the tiny Lithium 3.8V rechargeable battery and wired the DC output of the converter to where the battery was attached. I then attached a larger speaker than the stock tiny speaker (8 Ohm). Technical note: the board has a type 8002B audio amplifier IC chip. It is important to use a baffle (chamber) for the thin speakers that might fit in a S-gauge car; it doesn't have to be fancy as illustrated by the vitamin bottle neck. The video doesn't show it well but messing with the baffle can have a dramatic difference in sound volume and bass.
I confirmed there are buckets of "free" downloadable Christmas songs in MP3 format on the web. I placed a couple along with some other train related MP3 sounds to illustrate the concept. The photo shows the directory of files on the microSD card; I think it's common knowledge (?) that a convenient way to transfer MP3 files to a microSD card is with an SD-adapter as shown in video. Most, if not all, MP3 player modules simply cycle thru all the files on the memory card. In the video I am pressing the "next song" button to move things along! In this case you simply apply power and it just starts playing.
The crossing bell MP3 "song" was leftover from an OGR thread on a low-cost DIY crossing-bell sound accessory. It reinforces the concept that an MP3 player, typically associated with playing music, could be re-purposed for non-musical layout sounds.
