In addition to what Spikey and Shawn mentioned, I’d just add that multiresolution is almost certainly unnecessary here. Since you’ll get smoother results on the less detailed areas if the mesh is less dense, I’d recommend trying the Simplify brush with dynamic topology instead to set the detail to be much lower around everything except the mouth. Alternatively, you could keep your polycount relatively lower (at least far lower than 7 million!) and use the pinch brush to get edges in that area closer together without dynamic topology.
Hope that helps!
