UPDATE Aug 2017: The solution marked as accepted by me apppears to have worked until Ubuntu 15.10. For 16.04+ choose one of the more recent solutions.
For 16.04+ the config file is
/etc/bluetooth/main.conf NOT /etc/bluetooth/audio.conf .
I have a bluetooth speaker, Sony SRS-BTX300, which works in linux/ubuntu 13.04, but only after some fiddling. (Update Dec 2013: same problems remain in 13.10)
I have to set the preferred bandwidth mode to "High fidelity playback" (A2DP) each time after switching on the computer and reconnecting the speaker.
The mode resets itself to "telephony/duplex" (=low bandwidth) every time. It takes about 20 clicks to reset the speaker, (click through menu, disconnect, reconnects, select mode, test), and these are definitely too many clicks.
How can I make "A2DP" mode the default mode, an do so in a persistent manner?
A few screen shots will illustrate.
After choosing this menu by clicking on the bluetooth icon in the top panel in Unity...
After setting up this...
... the preferred mode will be reset to this after restarting/suspending the computer