The concrete slab floor with rat/frost walls (or something equivalent) are most probably a code requirement. Your post were put 5 - 6 feet into the ground to avoid frost heave and surrounded by concrete to avoid rot/vermin.
My suggestion for siding was made based on it being inside a pole barn. Pole barns aren't meant to be fully weather/vermin resistant (at least for more than a few years). What you're proposing would be nicer and certainly more sensitive to weather/vermin attack than whatever else the pole barn was meant to protect. Code prohibits untreated wood framing within 8 inches of the ground and siding within 6 inches to protect from weather/vermin. So while you may be able to back off roofing/siding the room for a while, some of these "real world" issues can't be avoided. The idea of building it separately outside the barn does make sense if you'll need the space inside the barn.
Building a toilet room could easily cost thousands (more foundation/walls/ceiling-roof, fixtures, routing water, 2nd water heater, avoiding freezing of water lines, septic tank/drain field or sewer line with possible lift station. Total cost would exceed the basic cost of your sound room.
Like Duke said, invest in the up-front planning. That's where most construction projects fail. Folks race towards half-baked concepts without fully knowing what they're doing or appreciating how much the "devil is in the details". Again bigger is better, avoid squares or parallel walls, stiffen the walls, add portable treatments as needed afterwards, and build conventionally (without shortcuts).
