I had a problem similar to this recently, it turned out I had the volume turned down on the N900 when in headset mode, the N900 stores different setting for different outputs, so check the volume when on a call through the car-kit.

Otherwise, the bluetooth drivers are in the kernel, it is unlikely that they would just stop working properly for no reason. Are you running the standard Maemo kernel or have you upgraded to kernel-power (if so which version)? I doubt it will help, but you can try reinstalling the kernel-modules and maybe the kernel too. It may be a problem with Pulseaudio, I'm not sure what you can do in that case, have you tried deleting all your Bluetooth devices and repairing them?

Failing that you may want to try flashing your N900 to get it back to stock condition.