I prefer the Etymotic in ear speakers for higher speeds and longer trips so I have removed my speakers and fashioned a little blanking plate to cover the plug hole they went into. I pushed an o-ring into the plug hole beneath the plate to make it a little water resistant. I may seal it over with some RTV but that's probably overkill since the plug is on the inside of the unit almost under my helmet. I attached a picture of the finished product to this post.
Thinking about it a little more I probably could have just jammed a little plug of rubber into the hole and dispensed with the o-ring, screw, and plate.