The External MIDI track in Logic activates the MIDI output from Logic to Reason only. You also need to activate an audio channel in Logic, where the audio output from Reason will go.
1. Open the Mixer in Logic.
2. From the Options menu, select “Create New Auxiliary Channel Strips…”
3. In the pop-up, select to create one new Stereo channel strip, and from the Input drop down menu, select Reason -> RW:Mix L/R.