Below are a couple examples of water-resistent audio gear. Like all Japanese electronics, this stuff seems to change weekly. The thing to do is google "marine audio" and look at a few sites. You'll pay about $550 - $650 for a CD/AM/FM unit plus remote control. If you get a car unit instead, you can get MP3. Personally, I don't think this is the way to go. You can get an MP3 unit and some Sony headphones for a lot less, and keep it in your tank bag or pocket. Or a mini-disk unit and a collection of disks - this is what I use, as it lets me record my records and CDs onto 3 1/2" optical media.
I don't know much about this stuff. There are driver-rider intercoms, which work with cords. Some allow music inputs, some don't. There are also bike-to-bike intercoms which work on either CB band or the new high frequency FM band. Some include music inputs, some include rider-driver intercoms, some use noise-canceling headsets. Some intercoms are stereo, some are mono. Perhaps some of you can email me with your experiences and suggestions. This list is a pretty poor attempt at this instant.
Charlie Grey's CB installation.
(custom) ear plug speakers, $150.
(generic) ear plug speakers, $100.