I found something that works for .wmv files

[The Reductionist]

After repeatedly getting Muxer error, I stumbled upon this combination of settings for the latest (2025 Jan 1) version of Avidemux on a computer with Windows 11 OS:

Video Decoder Lavcodec DXVA1
Video Output: Copy
Audio Output: Copy
Output Format: AVI Muxer

Outputs .mkv file

Have no idea if this helps anyone.  But, when you find something that works, tell the world!