Have you considered portable speakers that are battery powered (or passive) that connect via the headphone jack or ones that connect via Bluetooth?
For example:
Headphone jack, rechargeable battery powered...
I've also been looking for cases that provide minimal coverage of the ipad itself. The three I'm considering are:
Vaja Involution (~$150 from cases.com)
The Yoobao "V3/V4 Pdababy Magic Case" mentioned above (~$50 from ebay)
The Cyber Acoustics iPad Portfolio (~$40)
All of these have been...