iPhone 4S volume issues

I'm experiencing a bug when I plug my iPhone into my car stereo via USB. While playing music or a podcast, as soon as an email or text message chime or alert sounds, the volume of the music will suddenly jump up to an uncomfortable level and I have to rapidly turn my stereo down. Once the volume increases after whatever chime, it will stay at that level until I unplug the phone. I've checked the Music settings, and the EQ is off, Sound Check is off and Volume Limit is off. I used an iPhone 4 (and 3G) with this same car stereo setup and USB cable with no issues before.

Anyone else experiencing this? I'm thinking if this is widespread enough, it will be fixed with an iOS update. Then again, maybe it's just MY phone.