Linux

I've been using some ancient java app called jmkvpropedit to do this.

If you are talking about the mkv embedded title, try this:

find -type f -iname "*.mkv" | while read "i" ; do mkvpropedit "${i}" --edit info --set "title=" ; done

Seems like it has a CLI. You can figure out how to do this action with a CLI command, then do something like find -name *mkv -exec ... to execute that command for all the files.

Emacs Dired would be my goto here, though it's cumbersome if you dont know the bindings.

kill-rectangle and multiple-cursors within Dired are immensely useful

Edit: Oh, I just understood you want to mass modify the files themselves. In which case wgrep is useful here within Emacs, for modifying multiple buffers.

It essentially runs a grep command on a directory, collates all the results in a single buffer, lets you modify that buffer for all files, and then save in one go

Someone with more experience on sed or awk should chime in, but out of memory something like this (which MOST LIKELY WONT WORK, verify it before running it on anything important):

find -name *mkv -exec sed -e's/file=.*/file=' > {}.changed \;

That, at least in theory, reads every .mkv file recursively in a current working directory, finds lines that contain "file=" and replace that with "file=" and stores the output to .changed.