Ardour has two main settings which affect how monitoring is performed. The first is Edit > Preferences > Monitoring > Record monitoring handled by. There are two or three options here, depending on the capabilities of the hardware.

The other setting is the Session > Properties > Track Input Monitoring automatically follows transport state (auto-input).

Monitoring also depends on the state of the track's record-enable button and the session record-enable button, as well as on whether or not the transport is rolling.

If Ardour is set to external monitoring, Ardour does not do any monitoring.