I got a couple of these mp3 player boards off eBay recently. They were approx AUD $6.50 each posted. Deal Extreme also sell it.
It has a stereo 3.5mm socket for the audio output which drove my headphones (120 Ohms impedance) to quite reasonable levels.
Rx: 3.3V or 5V serial input.
Tx: 3.3V serial output.
Vcc: 5V power input.
Gnd: power ground.
The baud rate is 9600 8N1.
You can use pretty much any micro SD card.
I used a 32GB one for initial testing.
You can format it as FAT16 or FAT32.
The serial interface supports a reasonable number of commands, which are documented in the (Chinese) data sheet as well as in the code examples.
Speaking of documentation, all of the links for this on the various sellers’ web pages were dead. I did find it eventually and you can grab a copy of it here.
These modules would be great for hobby projects and there are a few Arduino examples easily found with a Google search.
BobNJ over on the Haunt Forum emailed me to say he got these modules to work in stand-alone and push button modes.