It sounds like these users are using the microphone in their cameras as their default recording devices. If they set the cameras as recording devices for communications, but the internal microphone as default device for other purposes, the webcam should not be used by Proximity.
See screenshots under the following help article to get an idea of how to do this. Ignore the rest of the article, just see the screenshots.
https://proximity.cisco.com/help/?platform=win&code=AUDCLNT_E_UNSUPPORTED_FORMAT
