Starting with version 4, the wave track is no longer a part of the MIDI roll view.
Orinj version 3.0.0
The purpose of the wave track is to allow you to insert wave files into the MIDI roll view. Suppose that you want to create a MIDI piano track that will be played within your wave session. In the MIDI roll view, you could insert a wave drum track. Then, when you add notes to the MIDI file, you can see at what times they should be added. You can play your new MIDI file with the drum track and be more comfortable that the MIDI file has the right rhythm to fit in your wave track.
The following is the wave track in the MIDI roll view.
To work with the wave track, use the commands under Wave in the Orinj menu. You can open and close waves and change the volume and pan of waves. The wave track is equipped with its own control panel, on the left in the picture above, which allows you to change the volume and pan of the wave, to mute the wave, and to select the wave audio output device. See also Orinj Working with waves in the MIDI roll view.
Using a wave file is not always necessary. If, for example, you set the grid tempo of the MIDI file (click on View and then on Show Grid At in the Orinj menu) and the MIDI tempo (with the MIDI control line) so that they are consistent with those of your multitrack session, you would be able to create MIDI files in the session tempo without the help of wave files.