Tab selects the next note. Tab selects the previous note. Tab or Tab adds the next/previous note to the selection.
While in mouse object mode, any note can be clicked to select it. Once a note has been selected, Left-clicking on another selects all notes between them. Adding or removing a note to/from the selection is done by Left clicking it. Clicking and dragging outside of a note rubberband selects a series of notes.
Three different selection operations are possible while in mouse range mode:
If Edit > Preferences > MIDI > Sound MIDI notes as they are selected is enabled, Ardour will send a pair of NoteOn/NoteOff messages through the track, which will typically allow to hear each note as it is selected.