Guacamole's protocol does already technically support audio streams in either direction, but this is only currently implemented in the server-to-client direction. The same support can be added such that client-to-server audio streams are initiated automatically (when enabled).
The procedure could be as follows:
- Guacamole client attempts to create an audio stream be sending an "audio" instruction to the server.
- The browser prompts the user for access if they have not already granted/denied it. If access is granted, the audio stream begins. If access is denied, the audio stream is simply closed.
This would require adding support for RDP's audio input channel (not part of RDPSND, but very similar). The same could no doubt also be done for PulseAudio.