There is no way to use the E17 as a DAC with that Sansa. All is not lost, tho. From what I've heard the volume controls on the E17 is superior to that found on the Sansa, so music would sound better when played at volumes that are only adjusted by the E17.
most likely you'll want close to 100% volume on your Sansa though........... that's what Bowei said. :P