iPhone 4 volume decreases at the beginning of some songs

My iPhone has started decreasing the volume of songs right at the beginning of the song. Within the first second or two of the song, the volume will decrease. Not completely muted, but enough so that you have to turn up the volume on the phone or the speakers if you want to maintain your regular listening volume. It's not every song, so when I encounter a song that gets slightly muted, and I increase the volume to compensate, the next song might come blaring out at regular volume. I have noticed this in my car via bluetooth, in my headphones, through the phone speakers, and through the external speakers I connect to (iHome alarm clock, etc). I have had the phone for about a year and the issue just started in the last few weeks. I have not been able to establish any kind of pattern (i.e. certain artists or albums or purchase methods) for the songs that get decreased.

Has anyone else encountered this issue, and if so, how can I fix it?

Thanks,

Jess