iTunes has an option which tries to even out the volume of all the songs in the library. Maybe you've got this turned on?
(I'm not sure if this would effect ringtones, but you could try it).
If nothing works you could also just download some third party software to increase the volume of the actual file on your pc.
Out of curiousity, how did you create these ringtones? The iPhone doesnt natively support mp3s or such. Theres a "trick" to create ringtones but in case you have used third party software theres a posibility this program lowered the volume, or it included an option for the volume you've overlooked?