I've been playing with mp3 players for swimming for the past 1 or 2
years, and finally have a set-up that works really well (i.e., easy to
use, sound is as good as regular dry land set-up). Here's what I
found:
For player, I recommend the waterproofed IPod Shuffle from
Swimman.com--cheapest solution is to get a new shuffle ($50 for a
refurbished one, or apply for certain credit cards and get one for
free), and Swimman will waterproof it for you for $75. Extremely
small, light, built-in clip, and it's solidly waterproof (e.g., can
get water in headphone jack without worries, and can use all controls
while underwater.)
For headphones, Aquapac earbuds are by far the best I've tried.
Excellent sound quality, and most importantly, they are very small, so
fit well and are not easily knocked out (other waterproof headphones
stick out from your ears in an awkward (and silly looking) way). I
use an Aquasphere "over the ear" silicon swim cap to add extra
security in holding the earphones in place.
The Ipod Shuffle clips nicely onto the back of the swim cap and you
can simply coil the extra headphone wire up and stick it under the
cap, so there's really no added drag as well...
Anyway, before I got to this point, I wouldn't have said it was worth
the bother to try to get music going while swimming, but as I said,
with these two products, it's very easy to set up and sounds great.
Best of luck,
Jon