Been wondering about this myself. Not so much for riding as when parked in the rain or washing the thing. More concerned with the front speakers 'cause they lay sort of flat and would seem to collect moisture.
It's rainy season here now and most days my bike is in the open lot @ work all day. If it's not rain it's dew heavy enough to puddle by the time I go home around midnight (soon will be frost/ice). I just leave the radio off for a while to be on the safe side but can't imagine the speakers not being designed for wet environments.
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