I had a knowledgable friend put whatever that black gunk they use to seal windshields with is called onto the metal frames of the woofers in my bass modules of my speaker system. Thought it made a slight improvement in bass clarity. He did the same to my midrange drivers in my satellites, but I couldn't tell the effect because we made a number of additional changes to the crossovers and internal wiring at the same time. We've done the same inside the chassis of my crossovers. While I think it can only help with components to reduce vibrations anywhere you can, I'm not sure it always helps to damp internal vibrations in a speaker enclosure, as it could be that the designer was taking them into account in voicing the speakers. What you're proposing will likely change the sound of the speaker; whether that change is for the better is not going to be clear until you try it, so maybe you'd be well advised to do something that can be undone if you try it (or contact the designer/manufacturer to get their thoughts).