Bleach might work; it's what I use to clean my rubber/plastic etymotic earplugs when they get dirty/stained. But it probably shouldn't be your first choice. Start w/ soap and water. If not, try baking soda and a bit of water. Or just hydrogen peroxide. Bleach will be the harshest of these chemicals, so try other things first.
Just make sure to wash out the chemicals from each previous trial so there's no chemical mixing . . . and don't use ammonia products if you intend to try bleach. Also when you work with hydrogen peroxide and bleach, wear gloves to avoid skin contact (especially cuts and mucous membranes).