how to record sound to an existing MP4 file

[Yasha Karant]

I am hoping that someone on this list knows of an application, 
preferably available for Linux, that will address the following.

Our university has forced us to use a compiled executable Zoom 
application downloaded from the university (and available for MS Win, 
Mac OS X, and DEB for Ubuntu or RPM for EL).  The user interface is very 
poor.  My wife recorded her class to the University-designated cloud, 
and I am not downloading the MP4. Unfortunately, because of the poorly 
implemented user interface, there was no indication that the audio 
recording was not working. As a result, she has the video but no audio.

Is there a method to run the video and at the same time (with minimal 
latency) record into the audio track of the MP4 as the video is 
playing?  That way she will not have to "re-record" her lecture, but 
simply supply the missing verbal content.  Any application for Linux 
would be appreciated; in a worse case, a MS Win one will suffice.  We do 
not have a Mac OS X machine.

Please do not reply solely to this list and my university email.  My 
private email -- much more reliable -- is ykar...@gmail.com .

Yasha Karant