In that price range, I bought an open-box logitech mm32 from the amazon marketplace. This speaker set gets returned a lot because it looks on the box that it will dock and charge an ipod, but in fact it does neither. Not that it matters if you are using it for a sansa.
Definitely nothing fancy (think of it as a pair of small PC speakers turned sideways), but loud enough and can run on either power cord or AA batteries. Comes with a fabric case to put it away in, too. My kids use it for audio books all the time.
Before you give up on your current speakers though, did you try changing the volume to “high” under system settings on your sansa?
