Nero Multimedia Suite still supports MMV format and with that you should be able to export the video to common AVI format.
Convert MicroMV movie to Audio Video Interleave movie.
We have found 2 software records in our database eligible for .mmv to .avi file format conversion.
Microsoft Windows software - convert mmv to avi on Windows
A multimedia converting tool for Windows
Convert from mmv file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned mmv to avi. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert mmv to avi on OS X
Mac version of a high-quality video converter, player and MPEG editor