We have a Japanese bicycle speaker the Otomo. http://www.crops-sports.com/en/crops/accessories/#105 (at the bottom of the page).
It's a 2GB waterproof and shockproof MP3 player that you can clip onto your handlebars. Awesome sound! We can hear it very well on the tandem or single bikes except when going 50km plus speed wise. Also you can use it with headphones off the bike or as a beach/BBQ music player. 9 hours run time.
Nice and small and doesn't take up a bottle cage!