I've done a little soundproofing over the years, largely to isolate the noise from an air compressor used for my tonearm. I used a product I bought from Acoustical Solutions in the Virginia area, a mass loaded vinyl/melamine sheet, sold in bulk, which is very effective. Not sure you would use that, but those folks do know soundproofing and sell the products- worth a call. I suspect you'll wind up needing a suspended floor, similar to what recording studios use but your needs may be even greater if you are dealing with neighbors who are hearing your music.
And the transmission of sound may be more than the floor- air systems, lighting conduits, any small opening or aperture is a potential leak. I found this with a small duplex sized cable pass-through, which I solved with melamine (Magic Eraser, originally meant as a soundproofing material). Doors are also "leaky" --you are down to the level of smaller than a "tiny bug" that can pass through. And, though I'm no expert, I suspect the walls, unless also concrete, will also act as membranes.
Often, people will hire an acoustician/specialist to do design and sourcing, which will add to the expense. Not an inexpensive proposition even if you source the materials without a consultant. You might give the folks mentioned above a call--they are professional, and can give you a rough idea of what this may entail.
I've known a few people over the years who leased or owned another space for their hobbies to keep them separate from their living quarters.
Be interested to see what you learn simply for my own knowledge.
Good luck,