What does the midi track trigger (synth, sampler)? Basically you need to insert a new audio track, and assign it to use the triggered device, program, etc, as the input source. Then arm the audio track and press record. The midi track should "play" the synth/sampler or whatever, and in turn that will be recorded to the new track.
Or, select the midi notes, and select both the midi track and the synth output track, then choose "Bounce To Tracks" from the Edit menu. This will create a new track with the synth output as audio.