I’ve called Apple about this too. Spoke with the a specialist in hardware and was told essentially that this most likely won’t be a priority for Apple to do anything about.
I have an iPhone 13, so no special audio to mess with. This is happening with ALL videos. Not just the ones I’ve recorded. Ones I’ve saved from messages, and Tik Toks that I’ve downloaded. Also, the mention of a speaker icon or button is non existent in my video playbacks. Again for both downloaded and recorded video.
The solution that has been working for me is to move the volume. I keep the volume turned up all the way, so when I was pressing volume up, it did nothing. If I press volume down at least once to move the volume slider, the audio starts playing. If you keep your volume level anywhere other than max level, pressing either volume up or down once will “unlock” the audio.
Once you have the audio playing on one video, it will work for all videos as long as you stay in the photos app. When you go to another app, the audio in the photos app will go back to being mute until you unlock it again.