.Glenn Barker commented
Animated images are widely used. Whether people like them or not isn't the issue, they are used everywhere and the gif format is too limiting. I understand that rebuilding the browser takes time and this isn't going to be high on the priority list right off the bat, but this should be somewhere on the road map.
WebM is in development – http://dev.modern.ie/platform/status/webmcontainer/
Opus is now in preview builds behind a flag. Please see https://blogs.windows.com/msedgedev/2016/04/18/webm-vp9-and-opus-support-in-microsoft-edge/
WebRTC 1.0 is enabled by default in Windows Insider build 15025. https://developer.microsoft.com/en-us/microsoft-edge/platform/status/webrtcwebrtcv10api