After l yawn my ears pop, and then it sounds like l have water in them for nearly half an hour. My breathing and speaking echo in my ear very loud, but very muffled or muddy sounding, while everything else is blotted out.

This also happens to me sometimes when I am working out or running - it just sounds like something fills up my ear and l can not hear properly.

If l inhale through my nose when my ears are like that, they will pop again very sharply and then go back and forth between clogged sounding and normal sounding.

but, if l avoid yawning, talking loudly, or breathing heavily, my hearing is above average.

What is going on, and more importantly, how do l fix it?

It is the pressure between your throat and your ear canals via the Eustachian tube (small tube connecting both).

When you yawn the air between the two becomes imbalanced.

Try holding your nose(so no air goes in or out) and blow hard into your mouth that is also closed. You should hear your ears pop back to normal then.