AudioTransmit API

Implemented api functions for audioTransmit and fixed related errors in functions for muting, unmuting and setting the microphone volume.
We probably need to be a tad more flexible in this API. For now it assumes that it's transmitting audio to an AXIS device, but the API doesn't really specify that. Perhaps an optional type parameter which defaults to 'axis', and we can extend this in the future if the need arise.

Also, it should be clarified that the sample url is for an AXIS device.