Most regular speakers you can find in stores require you to have them connected to a power outlet to operate at all times. Spending time in the outdoors with such speakers is no fun. But you could always rely on these solar speakers and some sunlight to keep rocking wherever you are:
Eton Rukus Portable Bluetooth Solar Wireless Speaker System: a Bluetooth sound system with a solar panel. It works with Bluetooth enabled smartphones, tablets, and PCs. It has an E-ink display for energy efficiency.
Honeywell Wireless Outdoor Solar Speaker: gives you a lot of flexibility when it comes to how you can power it up. You can power it up using sunlight or by AC power.
Eton Rugged Rukus All-Terrain Portable Solar Speaker: a new rugged and splash proof speaker system for mp3 players. It has a battery life of over 8 hours. It can wirelessly connect to any Bluetooth enabled device.
SoliCharger-SP – Portable Solar Charger with Speakers: a rugged, compact solar speaker for your iPhone 4 and iPhone 5. Weighs just 9 oz.
Goal Zero 19017 Speaker + Solar Panel: this is a speaker and solar panel bundle. You can charge other mp3 players with the included solar panel too.
These solar speakers allow you to take your music with you to the outdoors. They are not all cheap but give you more flexibility as far as powering them up goes.