At the risk of castering aspersions please don't do this to your speakers.
Is it OK to put casters on the bottom of speakers?
I have a relatively heavy pair of NHT 2.5i speakers that sound best when properly placed but are inconvenient to the rest of the houshold when not in use. Does anyone have experience putting locking casters on the bottom of their speakers. It would be nice to know how you did it without destroying the bottom of the cabinet.