Have you already seen this post Re: How to play audio on Edison - instructions, it discusses playing audio on the Edison board. Check it out to see if you find useful information, alternatively you can also post a question in that thread for more information.
Regarding the USB flash you would need to mount your USB device to be able to play the audio files. Take a look at mount - Linux Command - Unix Command for more detailed information on how to mount your USB device.
Regarding the MPlayer you can make one file play after another automatically if you specify it so. Take a look at 3.1. Command line in the “Playing more files:” option you can find one way to do this. I have not tested all of these methods but give them a try and let me know your results.