This is impossible to discuss with the information provided but I’m going to make a guess and throw out some info. It’s likely a 200A-240V-Single Phase main electrical service. The 2-100A-2P breakers likely feed sub-panels (garage, basement, pool house, etc.). Sounds like it’s a house with lots of electric appliances? The 50A-2P and 40A-2P breakers would be feeding condensing units, electric ranges, maybe a instant hot water heater…
You’re going to want at least one 20A dedicated circuit for your audio system. IMHO a single 20A will be more than enough but others here have different ideas. I would run a dedicated 20A-1P circuit with #10 (don’t go bigger, they will not fit the lugs on the breaker or receptacles - you could use #8s on the breaker and pig tail to #10s or #12s at the receptacle). At some point, those are large wires to install in a standard junction box. Heaven help you if you buy solid #8s as opposed to stranded. Also, 3#8s require a 3/4" conduit (if you live in EMT land).
I would run a dedicated hot, neutral and ground. Keep in mind, if you want a truly isolated ground, an isolated ground wire is required between the main panel, all sub-panels and the final branch circuit, and a true isolated ground bar in the sub-panels (they sit on rubber isolating feet). This is rarely done correctly in a sub-panel configuration.
One last thing, a disclaimer. All wire sizes listed above assume copper wire and THHN/THWN insulation. I live in an all EMT world (no Romex or NM: Romex is the brand and NM is the generic designation).