I believe the only wayto listen to multimedia i.e your MP3 files on the SLVR L7 is via the supplied the wired stereo headset or your own wired headphones using the adaptor.
As it only supports bluetooth 1.2 you only get the headset & handsfree profiles, which enable you to make/receive calls only via a bluetooth headset.
IIRC it would need to support the Advanced Audio Distribution Profile (A2DP) Bluetooth profile for you to be able to listen to multimedia using stereo BT heasdsets.