Hello milancro and Welcome!
I also have 512MB version of mp3 player (some noname brand

), but it plays fine and havent encountered something similar yet (I also hope I never will

).
But like always...there's got to be a solution to Your problem also...I am 100% sure about that!
So tell me few more things...
1. When You bought it...did it played songs correctly?
2. Have You reformatted the player or perform some other unconventional methods with it?
3. Does everything else works fine? And also...is this happening with ALL your songs or just some?
As far as I could read from other websites and forums...looks like your player has some problems with sampling....check few advices
HERE and
HERE and see if they could help you out. Also if some other members here have any idea about it...please help.
If you have a CD that came with a player...then try to see if that default song can be found there and try to play it...it should play correctly (or try to find some songs with various bitrates to see if some of them work)...otherwise...try to see if it has some options in menu to setting bitrates and sampling speeds....and if all that fails....maybe reformat it and install it from a scratch....something should work 100%!
Finally....I dont know if this software and stuff works on every flash mp3 player or not....but if nothing else wont work...maybe i can send you files and software that came with my CD, so you could check it out if it works for you....
Thats all....let me know about your progress....Good Luck!
SiliconFreak.