Okay, first off, how are you trying to "see" the music files on your Sansa?  Are you plugging the Sansa into your PC, then trying to "see" the music files?  Or, do you mean you can't "see" the music files on the Sansa itself (with it unplugged from the computer)?

 To "see" the music files on your Sansa while it is connected to your computer, connect your Sansa to your computer, open up the Sansa drive location, then select Tools (from the top tab), Folder Options, and View.  From there, choose "Show Hidden Files and Folders".  Your Sansa "Music" folder should then be visible for you to open up and view all of your music files on your Sansa.

Hopefully, this is what's causing the problem.

 P.S.  Make sure your Sansa is in the proper USB Mode in order to be able to open it up as a separate "Drive" within My Computer when it's connected to your computer.  Read your Sansa's manual as different Sansa's models handle this function differently.

Hmm, have you transferred everything as one huge "Playlist"? Have you checked the "Playlist" category to see if anything pops up there?

.wma? That may be the problem. Try connecting your Sansa to your pc, open up the Sansa drive, locate the "Music" folder and drag and drop some .mp3 music files (not .wma files) into the "Music" folder on your Sansa. Then disconnect the Sansa and see if you can see the .mp3 files.