It's not even a question about a loud environment, I an barely hear the one I downloaded in a quiet room. Additionally, on the only one I made so far it only plays the very first little bit of the ring tone and then the phone abruptly stops and goes quiet. The phone is still ringing but ringtone gone!
I made two ringtones this morning and the volume differs greatly between the two. I made one from You Can Call Me Al (Paul Simon) and one from Bleed It Out (Linkin Park). The Linkin Park one is VERY loud, it almost crackles the speakers in the iPhone. It kinda sucks, but oh well.
If you do a google search for "mp3 gain" there are some free software that boost the volume. If you make it too loud, it distorts it on the iPhone. I'm at work so i don't remember the actual program i used.