Hello nehringmike,
Thanks for posting in Apple Support Communities. Since this happens when you are connected using an HDMI cable, it may be best to first rule out issues with the cable or connection. If available, test another HDMI cable and port. Does the same occur?
If so, while connected, check your iPhone Control Center to make sure that your TV is selected for audio. You may need to touch and hold the group of controls to see full audio options as mentioned in Use and customize Control Center on iPhone - Apple Support.
You might also be able to stream audio using AirPlay. Try the following from Use AirPlay to stream audio - Apple Support.
1. Open Control Center on your iPhone or iPod touch, or on your iPad:
• On iPhone X or later or iPad with iPadOS or later, swipe down from the upper-right corner of the screen.
• On iPhone 8 or earlier or iOS 11 or earlier, swipe up from the bottom edge of the screen.
2. Touch and hold the group of controls in the upper-right corner, then tap AirPlay.
3. Tap each speaker or TV that you want to play the current audio to.
Take care.