It happens to my 01 UL just about every morning with the winter here in Minnesota, but the sound has never just disappeared after it was on in the first place. Was the sound audible as you had the key in the accessory position & then quit when you started it or was it audible after you started it & then disappeared?
So far, the dealers have just said that it is a problem with the amplifier, and that would need to be replaced (approx. $750.00). It really doesn't seem to me to be a problem with the amp if it works just fine during the warmer months & only does this disappearing act below freezing - I don't really feel like dishing out that kind of money just to have the amp replaced only to find out that isn't the problem.
No I had turned the car on and started moving. I noticed the sound went out as I was backing out of my driveway. I asked the dealer and they said they didn't know anything about it. As long as it comes back on I don't have a problem with it, well not if the fix is 750 bucks.