Audio to MIDI is difficult because any real audio signal contains many different frequencies so recognising the "main" note frequency isn't trivial. Also what does "minimal lag" mean to you (in actual time please)? What is the project to be used for?
And most Arduinos are too slow for audio processing though the MKR series are some of the faster ones. I'd build a single channel version and see if that will do what you want. Once you get there then reading more audio channels and assigning the output to different MIDI channels is easy enough. It just needs plenty of processing power.