You have to have some understanding of tracks and channels in order to handle voice assignments. A track is nothing more than a series or "sequence" of MIDI commands. A channel is the path that carries those commands to a MIDI device. A track can contain events that are directed to multiple channels and can change voices, but individual commands can have only one channel and only one voice at a time. It is usually better practice not to have multiple channels or change voices on a track, though. Your MIDI sequence will be easier to understand and edit if you avoid making them too complex.
To assign a voice to a track using Jazz++, click your mouse on the name of the track. It will produce a dialogue like this one:
You can change the name to anything you like. It is best to use the name of the instrument (e.g., piano) or the name of the part (e.g., melody). You can choose any sound you want from the list in the dialogue box. Below this you can set the channel number. Audio Track allows you to create sample sounds and incorporate them. That's way beyond this How To, but you can have lots of fun with it. Keep the "Force channel number onto all events on track" checked. Editing a MIDI track with multiple channels is frustrating at best and fruitless at worst. On this file, I am going to leave this track on channel 1 and select a piano voice, like this:
Here is the resulting change on the Main editing window after clicking on "OK."
Notice that there is a number 01 to the right of the Track label "Melody." That indicates that the track has been set to the piano voice. Now the melody should always sound like a piano.
A channel can techically carry commands for different tracks and sounds, but for practical purposes it is best to use a new channel for each new sound. There are exceptions for MIDI devices such as synthesizers that deliver a variety of output sounds on a specific channel, but editing is much cleaner if you use only one voice on each channel. Let's change the Alto, Tenor, and Bass tracks to channel 2. Simply click on each one and change the slider control to indicate channel 2. On the Alto track, change the voice to 20, Church Organ. You should end up with a result like this:
Notice that I did not assign a voice on the Tenor or Bass tracks. Since they are directed at channel 2, the voice assignment for channel 2 in the Alto track will be used. Play the midi file. You will hear the melody on piano, and the rest of the parts on Church Organ.
To learn about changing the volume settings, go to the next page, Changing Volume, or you can return to the table of contents.