Sound room flooring advice needed

You are going to need a vapor barrier before you do anything(concrete wicks moisture). Home centers sell an excellent product that comes in 2'x2' interlocking panels that leave you with a great subfloor on which you can use any traditional flooring. Conventional hardwood over this would make an exceptional base and floor for acoustics.

The good news is that any water issues would have shown up long ago. That said, I would treat the floor as though it was a shower base. They have a waterproofing that comes in a 5 gal buck and you roll it on, as waterproof as a swimming pool when it dries. Make sure your concrete floor is relatively flat and clean. Roll this stuff a few inches up the walls and you are ready to go with the Dricore.
There are several companies that make similar products to Dricore and I would check them also to get the best for what you need.
You can contact me by email if you have any questions that might not pertain to this thread.
Good luck with your project, do it right the first time.

Gluing to a concrete basement floor is a mistake due to wicking moisture. Floating floors work wonderful for room acoustics. Your always better off mounting turntable to studs. You can brace and beef up during wall construction so table is completely isolated.

Wrm57-
I have been involved with residential construction for 30 years and have built dozens of houses on the CT shoreline in the $1m range.
Is this a below grade basement? Are there any existing water issues in this basement or any drainage issues outside the foundation? water is always the biggest concern in a below grade basement.
If you live in Vegas or Az you won't have to worry about moisture but if you do. We use the 2x2 interlocking subfloor that is rubber like on the bottom and a osb on the top. You can put a floating (engineered)floor over this and your speakers will be solid as if they were on the concrete itself. I would mount turntable to stud walls as a matter of best practices.
I have used this product in the past and it's great.
Dricore

The engineered flooring will come with a padding that you can lay right over the Dricore. Lock all the flooring together leaving a little short all the way around for expansion.
Do all your walls and ceilings first, right down to the finished paint.
Install finish floor.
Install base trim and shoe mould to cover the expansion gap.
Enjoy for years.