Hi Paul,
I'm going on the assumption you tried all the possible software fixes before you resorted to replacing the speaker, a full DFU mode restore, for example.
So, given that, you definitely did the right first step in replacing the speaker. If we look at all the things that can go wrong, we've got the speaker which you've verified already, then the electrical signal path to the motherboard which is realistically the only other thing you could fix yourself. That would mean replacing the lightning port assembly that connects the speaker to the logic board. iFixit doesn't have a guide for that, but there's a Spanish site called Nadie Me Llama Gallina, or Nobody Calls Me Chicken that has exactly what you need.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/apple-5/apple-iphone-12-pro-1296/flex-de-carga-11575|Manuales / Apple iPhone 12 Pro / Load flex|new_window=true]
If it was my phone, that would be my next attempt at fixing the problem. Unfortunately, if that doesn't do it, you may well be looking at board level repairs that would take expert level microsoldering skills to fix. So I'd say give the lighting flex a try and let us know what happens.