Music synced with iTunes has a very low volume on iPhone? I noticed that recently when I sync my music from iTunes to the iPhone, the volume of the music is very very slow. 

For example, I can barely hear it while biking on the street, or if I'm in noisy environment.

For sure some albums were louder than others, but I believe in the past it was much louder on average.

thanks
You might have selected "Sound Check" (which will adjust your song volumes). Look at your iTunes preferences -> Playback to see if that is the case. Turning it off might help.