Sorry that you're having this issue with your sound! Can you please specify which sounds you are missing, or is the entire game silent? Are you encountering this issue with any form of headphones/airpods in?
Also please check if the switch to put your phone into silent mode has be flipped. You should be able to find this on the side of your iPhone 12. I recommend turning your volume up to an audible level during gameplay and flipping this switch in either direction to see if this is the cause.