Do you get sent folders full of audio files from other recordists to mix? If you do, I'll bet there are frequently stereo audio files in there that are actually mono. Apple's Logic DAW seems to be the worst culprit, organizing sessions in an "always stereo" way that confounds less experienced track exporters and leads to so many mono-as-stereo files. I've been putting NUGEN Audio's SigMod [Tape Op #127] on my master bus with a left channel polarity flip and mono sum on; then I import all the audio tracks, solo the stereo ones, and see if the track cancels itself out. If it does, I'll split the track into mono and then delete or hide the stereo track and one mono track. What a pain. Why would I go to all this trouble? In Avid's Pro Tools, we can just use their DownMixer plug-in to make a stereo track act mono, but we are still processing twice the information, and the file size is still double what we need – yes, I know this sort of grandpa-style worry matters less and less as computers get more powerful, but here's the real reason it matters to me – I am constantly panning tracks and looking for space in a mix, so a stereo file that is not stereo messes up my flow, and adding a plug-in to a stereo file to undo it seems pretty backwards.
So, here's MonoMate. It's a very specific app (not plug-in) that will check all the audio in a folder for stereo tracks that are really mono and convert them to mono. There's a previous app by Soundizers called StereoMonoizer, but it still hasn't migrated to Apple Silicon. Thomas Dulin wanted something quick and affordable to sort this problem out; thus, MonoMate was born. (Note that MonoMate is macOS 14.0 or later only at this point.)