Here's another iPhone trick where you can play the mono music through your existing Bluetooth headset with a simple little change in your audio output options:
1) Pair your Bluetooth headset with your iPhone - we're not gonna get into the specifics, just do it.
2) Go to your Visual Voicemail and hit the "Audio" button (when the iPhone is not paired with a headset, this button will say "Speaker."
3) Choose your headset in the subsequent audio selection dialog.
4) Get to your homescreen and hit the iPod icon.
5) Start playing a song and revel in the Bluetooth headset

Inform me if this been posted before and I will delete this post.