I bought a cheap Soken sound system for the bedroom, basically to add another couple of speakers and sub to the TV. It has a USB port and will play MP3 music without any problems, but I tried to play a couple of audio books, still in MP3 format, and it won't play them. What could be the reason? I checked on my laptop and the audio files play fine.

I found the answer. I reformatted the USB stick to fat23.