Two Way Audio:

Step 1. Hook a MIC into the Audio Input of the recorder (RCA)

Step 2. Wire from the Audio Output of the recorder (RCA) to a speaker. Easiest setup is to use a powered speaker that has an amplifier built in. Computer speakers would be a good example. 

Step 3. You need to go into the "Encode" settings of the recorder (Main Menu > "Camera" under settings > "Encode" > At the top choose the channel number you want the audio to come through (audio input number correlates with channel number) > Then you need to enable audio for both Main & Sub Streams by checking the boxes towards the bottom of the menu, you will see where it lists audio. 

Step 4. Use your Smart Phone and download the Techpross app and get the cameras up and running. You can use this video here for that step 

Step 5. Pull your two way audio channel full screen in the app, and press the speaker icon. You should now be hearing audio coming through from your MIC. To speak simply turn off the speaker icon, and enable the MIC icon and begin talking. 

If you are planning to do this from a PC with a Microphone plugged into your PC, steps 1-3 are identical. Simply rather than installing an app on your phone, download SmartPSS to your PC. From there Step 5 is the same.