iTunes volume abruptly jumping

I just noticed this on my iPhone 4 after updating the OS to 4.0.2 last week: When I start playing anything in iTunes (audiobooks or songs), the volume is very low and needs to be boosted way up to hear; but after an audio alert rings, the audio level jumps up to normal and I have to quickly turn it down. This happens both out of the headphone jack and out of the Dock. (I use a Dock connector to hook up to the audio system in the car.)

My workaround is to open the Settings>Sound and play the "Tri-tone" alert after starting iTunes. As soon as the Tri-tone finishes, the volume in iTunes will jump up to normal.