Hi Anya,
Welcome to Apple Support Communities and congratulations on your new iPhone 12 Pro Max! We know you have a lot invested in your new phone, and want to make sure that the volume level works the way you'd expect. So we're on the same page, is the volume too low when you hold the phone to your ear, when you're using speakerphone, with headphones, or all of the above? If it's happening while you're holding the phone to your ear, let's try working through the steps below and see if the behavior improves. In particular, you'll want to make sure that any screen protectors, cases or films are removed from the phone before testing.
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If the behavior is happening with headphones, are you able to test with a different pair of headphones to see if the problem persists?
We look forward to hearing back from you soon!
Take care.