I live in an apt. and would like to lessen the sound transmittance from my stereo speakers.
I have foam around them now.
Does it look like it does a good job of minimizing the sound being transmitted through my walls?
The best kind of foam to use for seismic isolation is a soft absorbent polyurethane foam which is dark grey.
The worst kind of foam to use is one that is stiff such as white styrofoam which conducts vibration efficiently.
Firstly, avoid stiff foam in favour of soft "mushy" stuff.
The sound vibration continues to be transmitted most efficiently through the nail/pin/clip attached to the wall support used to hang the speakers. There will be very little vibration transferred through the speaker case itself (which has been insulated from the speakers by the speaker frame.) You can verify this by touching the case near where you have foam blocks using your finger tips.
Secondly, If you also hang the speaker with an elastic bungee cord, much of the vibration will be absorbed before it can be conducted into the wall supports.
The way I understand the situation, the worst part is the speaker being mounted on the wall. The "hook" (or whatever it is) transmits most of the sound to the walls.
I can suggest two variants of the same basic idea:
Install the speaker on a TV stand, placing a soft foam between them. The drawback is that the speaker might not be stable enough and might fall.
Use a kind of a "claw" to install the speaker on the wall, each "finger" of the "claw" being wrapped / insulated with soft foam. Worst case, use a second TV stand installed upside-down to behave like another "finger", on the top of the unit.
In my old turntable DJ days a mechanical engineer turned me on to Sorbothane pads to stop my LP needle from jumping on every vibration. Love these.
Maybe you can adapt them to your needs.