We've had this same issue with our 2010 Tucson Nav unit. Occasionally, no sound output of any kind. Usually, turning the car off, then restarting would solve it, but one time, it just stopped totally. Took it to the dealer, who performed a "reset", and then wanted to charge me $70 for an hour of labor. I got them to cut that in half, but it still did not leave me a very happy camper.
There is obviously a problem with Hyundai's nav units, if this is still happening in the Sonata model as well. Next time, we will just do the battery and/or fuse pull to do the reset.
2002.5 Hyundai Santa Fe
2006 Kia Sportage
2010 Hyundai Tucson Limited FWD
2013 Hyundai Sonata SE