Actually, this isn't a question - it's a solution. I was trying to figure out which of my extensions was stopping me hearing anything in Collaborate ultra. It was "Disable HTML5 Autoplay" - which successfully disabled the audio, though not the video, feed. [GitHub - Eloston/disable-html5-autoplay: An extension for Chromium-based browsers that disables autoplay of HTML5 audio … ]
I'd realised it was one of my extensions, given that it was working fine in an extension free set up, and made a guess at that one.