I ask more verbose:

If you first make sure nothing (charger, computer, etc.) is connected to the usb port of the N900. Then power off by selecting power off from the power key menu. Make sure the device does really turn off and does not reboot immediately (as might happen if you used host mode). Then you turn it on in the usual way by pressing the power key.

Does it happen that the audio does not work after such a sequence?

If if does please run the command

dmesg > out.file

and upload the out.file somewhere I can see it.