Syncing Guide
In order to sync, one person will use the host in Terraria Midi Player in order to send a signal to Terraria Midi Player clients telling them when to play. Hosts can tell clients what song is next but note that this does not actually change the selected song of the clients. When a client is ready to receive a play signal they need to click the Ready button so that it says Not Ready. In order to send a play signal, the host just needs to press the play keybind and wait for the specified wait time to elapse. A client will not play a song if they are not ready. Hosts can also tell clients to stop too (assuming the client started playing because of the host) by pressing the Stop keybind.
Port forwarding is also required if you plan on using this outside of your local network. This may sound easy, however, syncing isn't that straight forwards.
Most computers' clocks can be off by seconds to minutes. One way to simplify this is to all use the same time server on your machines, such as pool.ntp.org. This will sync your times to within about a second of each other.
Set this when the time it takes to reach the clients is longer than the wait before playing the song. Usually the default 4 seconds should be more than enough. Next comes the actual work.
First, one person needs to assume the role as the intended recipient of the performance. This needs to be done when performers are a far distance apart from each other and the server. If performers are far apart, being in sync may require different time offsets for each person. Test your sync by both playing the same part of a song and checking the differences of when each player starts. The recipient will then determine the amount of milliseconds to offset each performer by in order to fully sync the song. Continue this until each player is on the mark with each other.
Syncing is one thing, but it may not be the end. Not everyone's hardware is the same. Some computer's clocks may not just be off, some may be faster or slower. Unfortunately there is no real way around this. The clocks should be good enough to stay in sync for the duration of a song but afterwards the time offsets may need to be adjusted again. You can try updating from your internet time server again before playing but there's no guarantee it will help.