I have the 1713 for less than a week now, had an initial issue similar to you with my iPod (shuffle). Note that before this unit, I had the 1613 then exchanged for the 1713 because it went on sale (for $30 difference, why not). I had no problem with audio coming out of the front USP port in the 1613 at the first try.
For the 1713, I "messed" around with the button to make it work, after a couple minutes of no sound:
- on the remote, press SETUP (p.83 of Manual)
- select Restorer submenu
- select Mode 1 (High) (p. 86)
Then I heard the intended music, a happy camper now!
Hope this help!