Why won't some songs work on my cell phone? Sandisk MicroSD 512 mb?

How do i figure out if a song is wav or mp3 on the computer? I believe all my music is mp3 but I am not sure.. When i place my sandisk 512 mb memory card into my phone it will not read all the music. What can I do to fix this? Someone please help!! Some of my music works only the songs i REALLY want wont work! haha