MUSIC ON SD CARD

I have just down loaded about loads of music onto a 2 gb sd card .via my card reader , but when i put the Sd card into my audi satnav / media player it recognises there is an sd card in slot one . but does not play any music . I tried someone elses sd card with music on and it read that .
DID i do something wrong . every tme I added a cd onto in the memory was going down on my computer so it was loaded onto the sd card .
any help would be appreciated