The best way to convert VP9 MKV to MP4 or AVI- Bigasoft Total Video Converter will show you.
Membrane on X: "Membrane development highlight: MP4 support in Membrane is in its final stage of development! 👉it will make possible to record WebRTC sessions to MP4 with H264+AAC. 👉WebM support coming
VP9 Encoder and Decoder: Convert between VP9, MP4, AVI, FLV, H.265, WMV