Sorry for the late reply, but you are sure to have much chances to recover those lost pictures and music! Try photo recovery software to deal with the situation. Here are some picture recovery programs for your option.
Photo Recovery (for Windows)
Photo Recovery for Mac (for Mac OS X)
Both programs are able to recover pictures, videos and music. You may rely on to recover most of your lost files on your Sansa Express.
How to recover: --> Install the software on your computer, then connect your SD micro card to start recovery (the card will shown as a removable disk)
Note : Do not attempt to put more files to the card before your lost items are recovered!