I burn music for using both CD-r & music CD-r.?

I found out when I put them into CD player with MP3 and they can be played no problem at all. But I put into CD player without MP3 then you will find only the music CD can be played and the other won't ( the songs I burn include both mp3 & wma files but only wma's song can be played). Can anyone tell me why? Thanks.