The videos linked in the comments of the top solution and the TA walkthrough (which are the same exact videos) weren't working for me. The background music was interfering with the midi and some of the midi tones were just too low that the game wasn't picking them up. I decided I'd make my own recordings to fix these two problems. My recordings aren't perfect, but they're good enough. I used three different devices to play them back -- my laptop, my phone, and my old phone.
https://1drv.ms/u/s!AkK1-N_CM-yWhZx2TxNnw950fglvXg?e=GhLil7The recording on the 2nd harmony part is delayed by about 5 seconds to make syncing the audios easier, so you don't have to press play on three different devices at the same time. There's also a second version of the lead recording that is delayed by 5 seconds, if you need it for some reason.
For pressing play on the recordings, I used the hi-hat intro to help with the timing. It goes "
1... 2... 1, 2, 3, 4". To time the 5 second delay recordings correctly, hit play on the first "2". For the non-delayed recordings, hit play on the "and" beat of the "4".
It took me around an hour of attempts until I got a perfect sync and unlocked the achievement. Good luck!