Hi Peter,
Since my last post I have systematically excluded all possible combinations of audio interfaces. In each case the audio passes through as desired however tethering is not possible. When any exclusion is applied the handset no longer registers correctly in the guest vm, and windows unable to apply a appropriate driver. The only condition where the handset successfully tethers is when it is unfiltered. When it is unfiltered, there's no sound!
If I manually connect/disconnect the handset via the 'Connect USB Device' menu option, then sound stops/resumes respectivley. I can do this repeatedly with the session and this behaviour continues. It appears that audio redirection to the local device is disabled when any USB audio device is presented, even if the USB device is not the selected (or default) audio playback device on the vm.