Same problem with mine, and it applies to a few different music players. It seemed to happen suddenly too. I started a bug thread but it seems to have been widely ignored. I have even got a new SD card and the same problem applies.
For some reason I have suddenly lost usual access to my SD card. So, the lists in the music app are empty, as is the gallery. I've just updated to 1.3.11 but there is no difference.
I have tried another SD card and the same problem occurs.
However, if I go through File Explorer I can...