In my quest for finding a reliable MP3 player that wasn't theft-prone (like the knock-off iPods all over eBay) but also wasn't super pricey, I stumbled upon this!
It's pretty self-explanatory. It's USB-powered, can read any MP3 off of a flash drive or SD card, and has an on-board 2 watt mono amp so you can drive a small speaker, and can AUX-out via 3.5mm. They're smaller than the size of a playing card, so hiding them in a prop is painless.
I'm sure many of you prop builders have these in bulk, but I think it's just about the best darn thing since sliced bread! Playback isn't 100% gapless - there's about a half second delay between tracks or repeat of a single track, but that's way better than the last player I bought (which retails for $70).
I purchased a set of 3 modules from the linked site above for $5 plus shipping, and it took almost 4 full weeks for them to be delivered. I'm using them in conjunction with a multi-zone amp, so I can use multiple modules across different zones. Honestly I was really nervous about the purchase because anytime I see raw parts, I feel like I'm not educated enough for qualified to be handling them but these turned out to be really great.
Thought I'd share!