Verify the file names. Remember, the file names need to be their designated number plus the ".mp3" extension. For example: 1.mp3 will work whereas Advertisement1.mp3 will not. Confirm this by looking at the contents of your thumb drive in Windows Explorer, and look at the file names.Be sure to click Tools>Folder options> View tab, then look for the "Hide Extensions for Known File Types" option, ensure that it is NOT checked. If you uncheck it, then click apply. The file should read "1.mp3" not "1.mp3.mp3." The Multi-Messager USB will ignore incorrect file names.(This is the Most common Reason a file will not play).
Verify that the USB stick is securely plugged into the unit. If it is, look at the status light on the unit, check to see that the light is solid. If it's blinking, then the unit cannot detect the memory stick or the stick is improperly formatted.
Ensure that the files are valid .mp3 files. If they were not converted and/or encoded as .mp3 files, but simply renamed, the unit cannot read them. If taken from an audio disc, make sure that the file was actually ripped and not just copied, as CD audio discs display files named "Track01.cda" and so-on, they're not real music files just track indication.
Verify that audio is coming out of the monitor speaker. If you have audio coming out of the monitorspeaker, but not the output on the back, the file is probably Stereo.
Ensure that the file names contain only standard English alphanumeric characters. Symbols, and language-specific characters such as "ñ" should be avoided.