When creating a MIDI track, you can use the "omni" check-box to allow the track to respond to input from any MIDI channel. If the check-box is unselected, the track will respond only to signals on its assigned MIDI channel.

In the matrix editor window, you can adjust the "velocity" (loudness) of a note by using the "Resize" MIDI Tool (see Section 8.3.2, “Use the MIDI Matrix Editor's Tools” above)

If you find it difficult to work with Qtractor's matrix editor, but you find it easy to work with LilyPond, you can use this to your advantage. LilyPond will output a MIDI-format representation of your score if you include a "midi" section in the "score" section. It should look something like this:

\score
{
   ...
   \midi { }
}

You can import LilyPond MIDI output in Qtractor. Choose Track → Import Tracks → MIDI.