I have long used some tiny speakers found on eBay. They are too small to be very loud but enough to get some of that hill climbing inspiration when you need it. After hitting about 17 mph or so they are drowned out by the wind. As such I use them only on occasion while riding or when relaxing on the trike at a rest stop taking pulls off a good cup of coffee.
I have typically had these clipped to my headrest. That meant I had to deal with the cord and the iPod shuffle I use, which was sometimes a pain. Lately I have been using a new setup - having the entire thing contained within the helmet.
The best thing about this setup is this: it's a "zero mod" change to the helmet. No cutting, etc was needed. Some simple Velcro - the hardcore industrial kind - holds it all in place. The tiny speakers weigh virtually nothing. My helmet is a Giro Xen and it has massive vents, so the small speakers fit just right yet do not interfere with the fit of the helmet at all.
The entire giddy up can be removed from the helmet in a matter of seconds. It sounds great for what it is used for and carries virtually zero weight penalty.
Ride (and rock) on!