I was wondering if there was a way to play music or the radio while the device is connected to a computer through the mini USB for charging?
The power pins on the USB cable are slightly longer than the data pins. If you insert the cable into the computer only far enough for the power pins to make contact, you can use the player while it charges.
Or you can just tape out the data pin on the USB cable, then it will only charge your device and won’t connect…this way you can still play music and everything while connected to your PC.
Not an answer to your exact question, but a mini-usb cell phone charging cable can be used to charge your sansa while you listen, and you dont need to leave your computer on! A quick look on froogle.com shows AC to mini-usb can be had for less than $6 shipped! Of course tape tape is cheaper.
There also are some power-only USB/mini-B USB cables out there (one came with a small, portable Belkin AC powerstrip/USB charger I purchased).