My earpiece speaker not working

You will, of course, want to try all the software solutions you can think of, as well as cleaning the speaker grille with a soft brush, maybe some compressed air, and some 90% or higher isopropyl alcohol.

If all that fails, the fault may well be with the speaker itself. On your phone it's part of an assembly called the Earpiece Speaker Assembly. The bad news here is that replacing that assembly will kill your Face ID and the only way to get it back is to pay Apple for a full screen replacement.

However, a guy on YouTube found that the speaker is only secured by two small solder points and replacing it by itself does not affect Face ID. I wrote a guide for that procedure on the iPhone X, but it is very much applicable to your iPhone 12. Start with your Earpiece Speaker and Front Sensor Assembly guide and follow it through Step 28.

iPhone 12 Earpiece Speaker and Front Sensor Assembly Replacement - iFixit Repair Guide

Once you get there, switch over to the iPhone X Earpiece Speaker replacement guide on Step 29. That will guide you through the two small solder points you need to unsolder to disconnect the old speaker and replace it with a new one.

iPhone X Earpiece Speaker Replacement Without Losing Face ID - iFixit Repair Guide

If you decide to do this, take pictures of everything after Step 28 and use my guide as an example of how to create your own guide so people who follow after you have a guide to go by.