madplay is a music player which can easily play all the mp3 files in a directory.
For instance, with raspbian, install madplay with "sudo apt-get install madplay".
Then you can play a single mp3 file with "madplay /usb/tunes/tunename.mp3", or all tunes in a directory with "madplay /usb/tunes/*.mp3".
(This assumes that you have put tunes in "/usb/tunes"--if your tunes are located elsewhere, replace with the appropriate path.)
Documentation here: http://www.linuxmanpages.com/man1/madplay.1.php