It depends on the design of the mp3 device. To listen to music the mp3 device must support a Bluetooth profile.
If it does...
To listen to music in high fidelity stereo, both the mp3 device and the headset must support the Advanced Audio Distribution (A2DP) Bluetooth profile.
If the mp3 device supports the A2DP profile, you can use it with a Bluetooth stereo headphone to listen in high fidelity stereo.
If the mp3 player does not support the A2DP Bluetooth profile, you will only hear the music in mono.
If the mp3 player supports A2DP but is also a cell phone, then it may or may not allow you to listen to music in stereo. It depends on which Bluetooth profiles the phone uses to transmit music, and this varies between phone models. You may hear the music in mono or in stereo, depending on the phone's design.
Here is a list of our A2DP headsets.
VoyagerPro+ & Voyager 855
Backbeat 903, 903+ & 906