Have the same issue with Xperia 10 & the Bluetooth Speakerphone in my car (Nissan car entertainment system), and also with BT Earpieces. Was also the same with my old X2. It works, but too low audio level for me to hear and the telephone partner also said I was too low. Therefore it can’t be a problem of codecs, but of audio level. Please simply boost up volume!
Q: In what system file is this defined? Please give information how this can be done by small system hack.