I searched extensively, and nothing really fit the bill like Midi mixer. Maybe midi2key, it looks more clunky to get up and running and doesn't seem to have fader support for application volumes, which is one of the main features of Midi mixer. I guess something like midi mixer could be made in autohotkey scripts, but not without a ton of manual coding and debugging. I'd count that as no real alternative unfortunately. Hoping we do get some update on the status of Midi mixer! It sucks that the project seems to be abandoned with no word from Jack.