Hi Terence,
Well, it seems to me you're on the right track; the earpiece speaker would be my first approach as well. The contacts for the speaker are on an assembly called the WiFi Antenna; those contacts run through that flex cable to the motherboard so it's definitely worth trying to replace that.
Of course, if it still doesn't work after that then you're probably looking at a motherboard/audio chip problem that would require specialized expertise to fix, but I'd definitely replace the WiFi antenna first. Unfortunately iFixit doesn't have a guide for that, but the Spanish site Nadie Me Llama Gallina has just what you need.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/apple-5/apple-iphone-13-pro-max-1324/antena-wifi-11954|Manuals / Apple iPhone 13 Pro Max / WiFi antenna|new_window=true]
You can buy the part itself right here on iFixit, and of course don't forget another sheet of screen adhesive so you can put it back together again.
[product|IF262-071|iPhone 13 Pro Max Wi-Fi Antenna|new_window=true]
I'd say give that a try then come on back and let us know what you found.