Since a passive speaker doesn't emit EMI by itself, I can't see how the FCC would care. FCC rules would cover the entire system, from amp to cables to speaker, if sold as a whole system. Since it would be unintentional radiator, certification is not mandatory, but you'd better be within the limits if they decide to look (quite unlikely) due to some incident or complaint.
There are probably a number of standards UL could certify a speaker to, like flammability and the like. However none of that is legally required. Some resellers won't carry products without UL or similar certification to particular standards, depending on what the product is.