It´s all about how low the resonant frequency of the system can become. I’m sure that Sorbothane works well to a point, but if you need proper decoupling all the way down to the cutoff of the speaker (if it plays at least somewhat low), you need specialized solutions. If the speaker has a useful response down to 30 Hz, the natural frequency of the system needs to be lower than about 15 Hz to offer decoupling all the way down.

AMC knows their stuff. Have a look and play around with their application (it´s free) and see what happens when trying to decouple a lightweight speaker with good results.