Hi, I’m new to the forum but thought I’d comment to see if it helps you in any way.
Firstly as you said expanding foam isn’t the best for soundproofing as it’s not dense, if you can use cement, for deeper holes use a mortar gun to squeeze the cement in.
We got a builder to cement render the internal walls to make them airtight and level but this isn’t a major issue if you don’t
We then built a stud frame using (4x2) and used a rubber strip on the outside to ensure it’s not directly touching joists/walls. If you can, don’t directly fit the frame to any walls especially the party wall, fix it to the floor + ceiling joists.
We researched lots of companies for the clips/bars and found they all pretty much have the same reviews/data but we opted for genie clips and bars as they were miles cheaper.
1st layer of 15mm acoustic plasterboard leaving a 5mm gap around the edges for the acoustic sealant (ac50), layer of tecsound sy100 on top of this, 2nd layer of 15mm acoustic plasterboard again leaving 5mm around the perimeter.
Also you may also want to make note about electrical sockets, you can either buy socket putty pads or an acoustic electrical box, both will help with airborne sound.
One last thing, radiators or TV’s, use extra furring bars/clips for these and don’t fix into the studwork. A lot of people underestimate these weak points and still have issues with sound not knowing these little things.
