My two Acer Iconia tablets have Dolby Digital sound. I'm happy with this
iHome iHM60GT 3.5mm Aux Portable Speaker. It produces big sound for its small size, and the price is reasonable ($20). You can daisy-chain speakers together if you want stereo, but I find the one speaker more than adequate.
And...if you eat Pringles, you can send for their free
"speaker can" (offer ends October 31, 2012).