Go into sounds, and make sure the sound volume is all the way up (it happened to me bfore...somehow it went down to a lower level).
Also make sure the ringer on the side is all the way up (not trying to be a ********* with that suggestion, but I have run into stranger things)
I had this problem at first, too.....at least I thought I did. The vibrate and volume do appear to be much lighter than my previous phone, but so far with the ringtones I have, i can hear most of them without any issue.
You can make ringtones form songs that are loud/alarming...that is what I have done for some (i.e the beginning of Time by Pink Floyd, with all the alarms....very noticeable).
This seems to happen to me all the time. When I go into settings I find that the "ring volume" seems to have reset itself to the lowest setting even after I previously set it to the highest setting. And then when I check the physical volume buttons on the side, the volume meter is like only half way when previously I set it to full.
Along w/ those annoyances and a slew of other small, but annoying quirks, I just can't wait until firmware 2.1 comes out.
If your iPhone is hacked (Jailbreak) launch installer and search for an app called "iAmp"
It will increase the maximum speaker output and doesn't sound distorted (except music, but you can adjust the volume easily for that) It's an awesome App.
What are you using to edit your ringtones? Like kisstine said, in most programs you can make the clip at a louder volume than the source that way it is extra loud when playing on the iPhone. I've done this with some of my softer ringtones and it has worked pretty well.
Same thing happens here, but this happened to me on my V1 all along. So not a new issue for me. I used to wonder how it was that I kept resetting the slider, but now that I see others mention it, then I wonder what the deal is to make the change permanent.