This is a complex situation which there is no one easy answer to, other than just not using dimmers. The sound system installation and wiring itself can be just as much to blame as the dimmers, lighting and how they are supplied.
Assuming your consultant knows what he is talking about, the additional 200A sub panel needs to be fed from the service not the existing panel. This is to minimize neutral noise that could be getting into the sound system via the shared neutral.
Does interference travel through the wire or through the air (RF)?
Primarily through the wiring and grounds.
-Hal
