Mux video and audio files to MKV without using mkvmerge?

I'm trying to mux a 1080p video I got from youtube to my own audio file (wav).

However when I load the MP4 video into mkvmerge, I get the error "The input file xxx.mp4 does not contain any tracks".

Presumably it cannot read the video only file for some reason. This appears to be a known issue and has yet to be fixed.

Are there any other tools which can achieve the same thing?

try to mux to a new mp4 or m4v file with yamb, the try to import into mkvmerge again.

If I had some gum, I’d chew a hole into the sun…

Yes, or simply demux the video stream.