Is there any way to directly access an attached device's (e.g a heaset's) audio in/out streams, enabling value-added sound processing? If so, are the available streams clear or encoded (e.g. G.722), or both?
Yes you can use Direct Sound, WASAPI to access a USB audio endpoint to capture or render audio. Plantronics SDK doesn't provide any functionality with respect to the Windows Audio stream the SDK only allows to bring the RF Link for wireless devices.