Silicone (not silicon, which is a completely different material) has been around for a long time. I have had some silicone instrument feet in my parts bin since the beginning of the 1980s. Silicone is all the rage at the moment, because high temperature silicone has appeared in kitchen products, which has raised its profile and made it a household word. One of silicone's useful properties is its tackiness, which makes it non-slip and also pick up dirt (which rinses off easily).
Silicone, urethane, vinyl, latex, natural rubber - most elastomers, in fact - will provide anti-vibration properties if used appropriately. The choice of material and subsequent design will depend on the specific application. Sorbothane, which is a proprietary name for a form of urethane, is particularly well-regarded for audio applications.
Contact area is certainly important. If the contact area is small, the material may squash too much and become essentially solid and therefore useless for our purposes; if the contact area is large, it might not squash at all (in which case it is already essentially solid). So experiment with different elastomers, different contact areas, and different numbers of pads until you find a combination that deflects under the weight of your equipment by no more than, say, one third of the unloaded height. Put them under "hard points" like the corners of speaker cabinets; don't put them in the middle of panels where the vibration is likely to be greatest.
In the industrial world, the design and choice of AV mount is extremely complex. Some of the considerations are: the mass to be supported, the exciting frequency and its amplitude, the resonant frequency of the final assembly, and a multitude of environmental considerations (this is far from an exhaustive list). The most effective mount works best under one specific set of conditions; beyond that, it's always a compromise. If you're trying to isolate your loudspeaker from the floor, you're probably trying to do so across a range of frequencies and for a few resonant frequencies in particular. Any AV mount is going to be a compromise. If you want to get close to perfection, try suspending your speaker from the ceiling in a cradle of high test fishing line! Make sure you find a ceiling joist of roof truss first.
Before you try this, here's something else to think about. Our main problem is isolating the cabinet vibration - caused by speakers themselves - from the rest of the world. What if your speaker cabinets are perfectly dead, i.e. they don't produce any vibration? If this were the case, you wouldn't need any vibration isolation! While few (if any) speaker cabinets are perfectly dead, many of the better designed ones should be getting quite close. So at the risk of starting a wave of outrageous comment, I'm going to suggest that the poorer your speakers, the more you need resilient mounts. Putting it another way, if you hear a big difference when you put your speakers on anti-vibration feet, you might consider buying (or building) better speakers. If you don't hear a difference, congratulations!
Nothing wrong with being a cheap sceptic!