A couple of very good factors there.
1 - Poured concrete, rather than block, high density, no let through of high frequencies, only thing to watch is low frequency resonance. That is of cause just considering sound transmission.
2 - Sand. Sand dampens brilliantly. Please please make sure you are there when they do the back fill. If possible, make sure there is a layer of just rough sand, without back-fill and building debris, against the wall I have encountered a huge problem with a place I designed in Europe years ago, with one of the studios walls located in a basement transmitting low frequencies like mad. We really did not understand this - as it came from one side of the building only. In the end all I could do was advise them to excavate that side of the building........... and found that the builders had used the fill as a dump. Old pallets, pieces of duct etc., all against the wall, making the wall into a pretty effective transmitter, like a drum head.
Just to go against the grain frederic...
Rural is good - if you're in the middle of woods with nobody anywhere near. Most semi-rural places have increased soundproofing requirements. No urban noises - and at night sound can carries for miles!
Temperature - being underground, well insulated, have you considered your heat and AC requirements?
9' ceiling height - is that clearing without calculating floors and ceilings?
The walls. An 8" concrete wall (make very sure they put a good moist barrier in!), with a 2 x 4 timber frame inside. A number of things;
1). Back to the moist barrier again. Don't take risks here. I'd fit an additional layer of AST30 (thick asphalt impregnated roofing felt) against the wall before anything else.
2). Make sure the timber frame is not directly attached to the concrete wall, leave a small gap. Sampson galvanized steel ties are perfect to attach the frame to the wall, they do not transmit vibration.
3). With a concrete - sand - timber frame with insulation construction and its usual characteristics, this is one example where I would not use a drywall - resilient channel - drywall interior wall construction, especially if you ensure the frame has no direct attachment to the concrete wall other than by means of steel ties. Instead I would recommend the following:
One layer of Georgia Pacific Toughrock Sound-deadening Gypsum Board. This is thin, 1/4" ASTM C442.
One layer of standard 1/2 sound board.
One layer Georgia Pacific Toughrock Fireguard 5/8"
You can find details about GP products at
http://www.gp.com/gypsum/products/specialty.html#sound
I have found the above sandwich construction to be highly effective and also very cost effective. You would find a sandwich wall like that to be more effective for your particular outside walls.
The 1/8" soundblock Frederic mentioned is mineral loaded vinyl, sold by acoustic companies like Auralex for much to much money.
Because of your concrete wall, sand and internal wall, you won't need it in the exterior walls.
It is a very effective barrier material, with a claimed STC rating of 26 to 27. BUT - I have found this rating to have one major provision - it is only so if the material is floating. As soon as it becomes part of a ridged structure its effect is reduced, which really is normal. Use it for instance with a thin layer of high impact foam on either side, as an underlay, and its brilliant. Use it without foam as an underlay - you're better off with the foam only at 10% of the cost. Great stuff overall, if you need any ask me, as I can order direct from a manufacturer, which is at least 50% less than an acoustics company charges.
Now I'm out of time, if you like I'll do another one on your ceiling and air, which are going to be your primary challenges.
