Soundproofing with 16kg/m2 rubber panels.

When it comes to adding weight to walls or floors, it is important to ensure that the structure can support the added load. In your case, adding 220kg to 310kg of weight to the party wall could potentially put a significant load on the wall, and it is essential to check that the wall can bear that weight. To assess the load-bearing capacity of the wall, it is best to consult a structural engineer or a building contractor who can provide expert advice. They can assess the condition of the wall, the materials used in its construction, and the overall stability of the structure to determine whether it can support the added weight. Regarding soundproofing, the M20 rubber panels you mentioned can be an effective solution for reducing sound transmission. However, it is important to note that they work best when used in conjunction with other soundproofing techniques. Adding a layer of acoustic plasterboard on top of the M20 panels is a good step, but adding a second layer could be even more effective. Additionally, sealing any gaps or openings in the wall with acoustic sealant can help prevent sound leakage. Another option to consider is using resilient channels, which are thin metal channels that are attached to the wall and provide a gap between the wall and the plasterboard. This gap can help reduce sound transmission. However, resilient channels can be tricky to install correctly, so it may be best to consult with a professional. Overall, I would recommend seeking professional advice to ensure that your soundproofing solution is effective and safe.

As Mike mentioned, it is imperative that you patch any holes/gaps behind the plasterboard as this is your primary source of sound leakage. As this is a 1910 house it is unlikely that you will have a cavity between the walls but it's best to check this as the cavity reverberates the sound which can't simply be reduced by adding sound-insulating panels. Also, check with a professional if the floor joists could be exacerbating the problem. If the floor joist are connected, there is a possibility sound could be travelling from under the floors as well.