How can I reduce the volume of music on my iPhone?
Without resorting to using an audio program to normalize the tracks  (which you might eventually have to do if you want to make it “the right way™”), you can select the track(s) you want to lower in iTunes and CMD + i to “get info”, go to the Options tab and manually reduce the track’s volume.



You can have multiple track selections and the adjustment will apply to all the selected tracks.

I am not 101% sure if this is carried over the iPhone/iPad (although it should). If that’s not the case, you will need an Audio Editor like Amadeus Pro (there might be others, I happen to have that one and it’s very good, but might be overkill for what you need).

In your favorite editor, open the mp3 (or whatever format you have) and find the Audio Effect usually called Normalize and lower the dBs: 



If you have to modify a lot of tracks, Amadeus Pro has a nice Batch Processor which is some sort of “automator” home made. 


  The powerful batch processor allows you not only to convert large numbers of files between any of the supported formats, but you can also instruct Amadeus Pro to apply any sequence of sound effects.


See how I have added a Normalize −15dB to the “actions”.:



note: I don’t work for HairerSoft or Amadeus. There might be other editors (some freeware) that do the same thing, but the idea is the same. I know this one does it and excellently because I have it. :)