My stoker is my 8 year old daughter. We just started doing 20 mile rides in the morning. She recently started listening to music (thank McDonalds for Kidz Bop CDs in happy meals the past couple months). I'm not a fan of headphones while riding, but external speakers at a moderate volume is certainly acceptable. I always carry my iPhone with me, which has an iPod built in. I own a Creative Sound portable external speaker system that takes 4 AAA batteries I have placed in my handlebar bag before, which works well. I'd rather have the sound back by her, so found a little 1 watt Macally portable speaker for iPod (IP-A111) that fits nicely under my saddle. I have two small velcro straps that hold the speaker to the saddle rails. The sound output is as loud as I would want it to be. Take a look.
I also had to buy a 6ft extension wire, outlined in red in the Photo below.
Woo-hoo! Portable sound to go.