He may be doing what we call a floating subfloor over the concrete. It is two plywood layers attached to each other installed on a diagonal to the room walls. It is not attached to the slab in any way, which helps keep impact noise from migrating to the unit below. The flooring is nailed or stapled to the plywood. That is the way I would do it. Key to success using that method is that the plywood needs to be quality and dry. I would not specify cheap pine plywood for that kind of install. You want a sheet that is dry, flat and stable. But, of course, your GC may have something in mind altogether different. As long as your slab is reasonably flat, it does not matter whether or not small dents and dings are filled prior to installing the soundproofing material. That is my opinion...some others may disagree.

So I am going to answer the question first and then explain the answer later. It would be very expensive to keep all the flooring the same height without adjusting the floor depth. Serenity mat in your entire house is not going to really do it, and the cost of full sound proofing the entire floor is quite high and unnecessary. Many of the solutions such as Serenity mat are largely made for retrofit applications. It is much cheaper and better to properly design your space than it is to buy stuff to put on it. Background: We are only concerned with limiting bass, highs will automatically limit as we limit bass. Bass waves do work. There are two ways to deaden bass travel. The first is to limit the work - this is what all absorption products do, any compressible material will work, you don't need to spend the money for serenity mat in fact a lot of people (myself included) use horse stall mats from Tractor supply as a cheap and easy alternative. The second way to limit bass travel is to increase the work required, simply add mass. The best solutions will use both. For the serenity mat to work well you really need layers that look like subfloor - green glue - serenity mat - green glue - 2 layers of 3/4 plywood glued together - flooring, also nails into the plywood shouldn't penetrate the mat. This creates a big mass over the mat and limits the absorption needed by the mat. However you are still going to have sound travel above the room and through the walls. Making sure you joists don't create sound travel channels and soundproofing the garage ceiling will be a lot cheaper than sound proofing the floor. You can get away with a Mooney wall type garage ceiling (with double drywall) and Mooney walls (with double drywall) in the garage and essentially eliminate the bass travel and spend a lot less than you would on the mat. To be fair attic and HVAC travel is going to be a lot more significant and likely than bass gain from a garage. I run two SVS PB 13 Ultras for Home theater and two Velodyne DD18+ for music. Occasionally I will throw them all on for a movie with a good soundtrack. I doubt your husband gets more bass than that. The only thing I use between our theater and the living room above it is 2 layers of drywall glued between the joists along with sound isolation clips with 2 layers of drywall hanging from them. With all 4 subs going you can barely feel it in the living room (during big explosions you can feel a very low deep vibration probably in the 20 - 30Hz range (which you are not going to get rid of.)) So back to your question. You are talking a lot of glue, serenity mat and plywood to get the floor the same height. It is just not worth it.

"Sound proofing" is not a realistic goal but reducing sound transfer between rooms relies on 4 principles: 1) mass 2) resiliency 3) absorption 4) crack & joint sealing Mass is achieved with masonry or thicker than normal drywall. Using a thickness on one side that is different from the other side also helps. Resilient materials and devices dissipate sound energy. You can use common or proprietary resilient metal channels to support drywall, drywall containing a resilient layer or gobs of green glue between 2 layers of drywall. In a floor, a thin resilient sheet will reduce impact noise. Absorption involves putting insulation in the cavity through which sound must travel from a crack on one side to a crack on the other side. It works when the sound must travel the long cavity dimension through a lot of insulation but it doesn't work as well in the short dimension unless the thickness is a foot or more so fiberglass is as effective as any other kind of insulation although some contractors will disagree. Crack & Joint sealing involves caulking the perimeter of wall plates and the edges of drywall panels as well as any penetrations. To achieve serious sound reduction in walls, studs and drywall should continue up to parallel joists, blocking between joists or to the floor deck above. Reducing sound transfer through a wall with a door in it requires a carefully installed acoustic seal on a solid core door with a drop seal at the floor which is generally impractical so the above principles aren't effective.

You need to find a blown insulation with ACOUSTIC ratings. And there's the rub. Most homeowners hear the word "insulation" and think thermal. But when they want acoustic insulation, they also think "insulation". And when the speak to THERMAL insulating professionals the word "insulation" gets misinterpreted (by both parties). The DENSITY of the material and the VARIABILITY of the particle sizes will dictate how much acoustic insulation you will get. Here's an example: A friend of mine used the acoustic calking in her basement ceiling. She then stuffed the ceiling joists with a SINGLE layer of Roxul Safe N Sound - even though I SPECIFICALLY stated she needed to DOUBLE stuff. Then installed acoustic channels and then regular drywall. She wishes she had listened to me about the double stuffing of Roxul. I didn't take the time to listen to a sales guy talking about the actual numbers for cellulose insulation, but here's a link that might help. https://deenergyconservers.ca/sound-properties-acoustical.htm I'm always suspicious of lab tests because the manufacturers will pay HUGE amounts of money to FILL the testing lab with thens of THOUSANDS of dollars worth of building materials so they can get 'massive' numbers from 4inches of blown insulation. They then publish the numbers for the ENTIRE assembly (could be 3-4 FEET worth of material) and claim it is the rating for their 4" worth of insulation. It's a trick I REALLY HATE. That's why I only look at Delta IIC ratings.