Grounding a 30KVA isloation transformer
Your situation is standard.  Unless you have a specific permit allowing a floating Neutral on the secondary of the transformer, you MUST bond the Neutral connection on the transformer secondary to Earth ground.  This is a simple bond to the case of the iso transformer and the incoming Earth conductor on the feeder provides the ground.

There are specific conditions where the Neutral is allowed to float.  Primarily in recording studios and only with single-phase 120V secondary windings.

The reason that the isolation transformer is helping you with your audio and video power is that you are creating your own Neutral on the secondary of the iso transformer.  The incoming Neutral is polluted with dimmer noise.  Your isolation transformer allows you to create your own Neutral which doesn't have all that noise on it.