"Not loud enough" could be due to a number of factors. For example, you are using a headset without noise cancelling. This makes your ear take the full brunt of the phone's maximum volume yet not hear the music very much because of ambient noise. This is actually damaging to the ears as well, since it puts too much strain. Get an earplug type headset, one that has sealing for removing ambient noise, or one that has noise cancelling features.
Another possible factor is that the headset itself is crappy.
Honestly as a medical student though, I'd advice a hearing test if you are on maximum volume and you feel its not "loud enough", even on earphones with no noise cancelling, just to be on the safe side. There may be an issue.