Any idea what the substance is? You might try some tar remover from an auto parts store. It is safe for car paint, but not sure about your speakers. Like mentioned above, I would try a small inconspicious spot first to see the results.
How do I clean a sticky speaker
I just picked up a pair of Celestion SL600 speakers that are covered with a tacky black substance. One speaker is so sticky that dust and crud has stuck to it. I'm thinking that I want to strip off this gook and repait the aluminum cabinets. Lacquer sor of tatke of off but not easily. Any thoughts would be appreciated. thanks much