Weird Radio Volume Issue

I have a 2007 Elantra SE. The radio has recently developed a weird problem. I almost always listen to my iPod via a cable in the aux
port. The volume is supposed to slightly lower when the car slows down and stops, and then increase when you accelerate. Now, the opposite is happening: the volume increases when the car stops and decreases when I accelerate. I'm going to ask about this when I bring the car in some preventive maintenance in a few weeks, but I was wondering whether anybody else has had this problem (and also whether it's covered under the warranty). Thanks.
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