Music files not showing in stock player with Galaxy Y
Check first your microSD card if it is properly mounted and most importantly have .mp3 in your Music folder. 

I had far too many instances where others copied only the shortcut of the mp3 file to their folder, then came looking for me telling that their phone does not detect the music.

The folder where your music is stored also must not have a .nomedia file present or all files will be ignored.