b/w = between
msc = mass storage class
mtp = media transfer protocol
Msc and mtp are settings that can be toggeled on the player itself by going to settings>usb mode. Which ever one is checked is the mode you are using. try searching for your files using both modes, one after the other.
how did you transfer your music? was it all done on the same computer?
a possible alternative is to format your songs and reload it in msc mode, after transferring, verify you can see your files to avoid running into this again. I personaly prefer that over searching and searching for my files. Just makes it convenient so I dont have to fiddle with it everytime i wanna add/delete stuff to the player