You hate when this happens and it happens all the time. You're trying to watch a movie, either at home or in the theater. The action sequence is going on and you're nearly deafened. Then the key dialogue starts. “What?! What did she say? Why are these people mumbling? I missed what the hell they said, don't actors use diction anymore? ArGH! I don't know what the heck is happening!” You hang on and tough it out. If you're at home, you do the ritual of turning it up when people seem to be talking about something and turning it down when the music or sound effects blast out of the speakers like a particularly abrasive guest that is intent on crushing everything around them. It's not your fault, you do not suffer alone. It's bad audio mixing and it has got to stop. You see, when people mix audio, they're looking at the whole… well, picture. Sound sets a mood, sound is the finishing touch on a production that when it works well, is nearly invisible, but when it doesn't, it's the mountain high zit right on the nose.
Unfortunately, subtlety sometimes is removed from sound mixing. Go for a walk and listen to everything around you. Sound is layers. And as you pay attention to those specific layers, they become, oddly, louder without an increase in decibels. These days, audio is LOUD, with everything seemingly pushed to the level 11. The art and beauty of sound is often lost within the process of sound for production. And when it comes to dialogue, it can be painful. Just because the dialogue is clean and audible in headphones, does not mean it works in the final. What to do? Adjust your method of listening. Take off the phones and listen as an audience member. Can you really hear the words being said clearly? Is the shift from music and sound effects to speech smooth? Are the layers of sound working with each other to create the full experience? If you were trying to follow the plot, would you throw something at the screen because you can't hear a damned thing? Then go back and adjust it again. Everyone has to do it. Your sound may not be as big and impressive, but it will work better for the audience.
Once that mix is right, export the track and augment that if you really want it to be loud.
Sound is a very simple thing for production. Make it clean, clear and time it to the action. The art of sound is to create an interwoven auditory experience that approximates the complexities of reality and serves the needs of production. Plus, some of the actors do mumble, but we'll fix that in post.