Hi Scott,
 
This is a good one! Here are Amazon’s list of steps to troubleshoot:
 

• Verify that your devices are compatible with home theater mode (I put a list of compatible devices below.)
  
• Verify that your Echo speakers are “online” in the Alexa app.
• Connect all devices to the same Wi-Fi network (make sure they’re all on the same 2.4 GHz WiFi network, not 5Ghz.)
  
• Turn up the volume on your speakers.
• Play a video title from a different streaming media app.
• Make sure to select the home theater audio option on your Fire TV to match your activity. For video playback disable Gaming Audio. For gaming, enable Gaming Audio.
• Restart devices in your home theater group. For Echo devices temporarily unplug them from power. For Fire TV, go to Settings > My Fire TV > Restart. Wait a few minutes before playing content.

If the issue persists, delete the home theater group in the Alexa app and set it up again.
 
Compatible Fire TV Devices

• Amazon Fire TV Stick 4K
• Amazon Fire TV (3rd Generation)
• Fire TV Cube (1st Generation)
• Fire TV Cube (2nd Generation)
• Amazon Fire TV Stick (3rd Generation)
• Amazon Fire TV Stick Lite

Compatible Echo Devices*

• Echo Dot (4th Generation)
• Echo Dot (3rd Generation)
• Echo Dot (4th Generation) with clock
• Echo Dot with clock
• Echo (4th Generation)
• Echo (3rd Generation)
• Echo (2nd Generation)
• Echo Plus (2nd Generation)
• Echo Plus (1st Generation)
• Echo Studio

If you’re still having trouble, I would recommend reaching out to Amazon directly.

Hope that helps!
 
-Daniel