There are some solutions on the market that contains Cetrimide and Chlorhexidine out in the market, which are used for first aid in most of the cases. Get one like that in soak it with a cotton wool. Gently Rub the soaked cotton wool on the earphone, and let it dry. That should do the trick.
Because of the large number of disinfection products, consult your pharmacist before applying it on the earphones in any case.