Diagnosis
This is almost always a permissions issue on your phone. The Lorex app needs explicit access to your phone's microphone to transmit your voice out through the camera's speaker.
How to Fix It
Go to your phone's main Settings, not the Lorex app. Find "Apps" or "Application Manager," then locate the "Lorex" app. Tap on "Permissions." You should see entries for Microphone and Audio. Make sure the toggle for Microphone is enabled (green or on). Once you've done that, force-close the Lorex app and reopen it. Try the two-way audio again—you should see a microphone icon animate when you hold the talk button.
If That Didn't Work
Double-check within the Lorex app settings for that specific camera. Some models have a separate audio settings menu where you can adjust speaker volume or enable/disable two-way audio. Make sure it's not muted there.
Heads Up
The speaker on these cameras isn't concert-hall loud, especially outdoors with ambient noise. The person needs to be relatively close to the camera (within 10-15 feet) to hear you clearly.