In order to be able to play audio or MIDI files they should be opened in a track. To play a track you should activate its Play button (it turns green) and start the transport.
Tracks have mixing and effect processing capabilities.
You can expand the mixer by adding any number of Group or Effect Return sections.
All Tracks, Groups and Effect Returns are routed through the Master section.
If a track's Play button is engaged and the transport is running, all streams in that track will be played.
If the Instrument slot contains an External MIDI Instrument the track's output will be sent to the corresponding MIDI Out Device. In this case the track is not affected by any Group, Effect Return or Master sections.
If a software instrument (Sampler, VST Instrument etc.) is being used the track's output will be routed through any mixer sections, and eventually be sent to the Audio Out Device, as if it where an audio track.