Why such measures for a space over a garage with no shared walls?

A threshold is a weather feature at an exterior door. For an interior door what you need is a tapered saddle sometimes called a reducer strip. You could also lower the floor framing over the garage.

Are you concerned that the sound would be a problem in the garage or outside?

You could save a lot of money by using resilient channels at the ceiling below. The things you suggest are usually for retrofits or to save space.

tapered saddle, got it.

I will ask about the floor being dropped down in the garage instead but I think that may depend on the direction of the joists, right?

The sound system my DH has is nuts. It's just ... very loud. The bass is very very obtrusive. I don't want it echoing through the garage into the rest of the house OR outside.

Thank you for your help.

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.