HDMI no audio on HD

I have an iPhone 12 Pro with several movies downloaded. I want to mirror this onto a screen using the HDMI dock connector, as trying to mirror over wifi results in an error message.

When I plug the phone to the adapter and to the television’s HDMI input and play either a television show or Standard Definition movie, audio and video play as it should. However, the movie I want to play is HD and when plugged in, only plays video and no audio. The phone is not muted nor the tv.

I would simply replace the HD file with the SD file from the iTunes store or an old laptop, but whether I download or transfer the version I get is always HD, even with the iPhone’s TV setting for purchases/rentals set to Standard Definition.