Have you corrected the issue with the ringtone volume? If not, make sure the Ring/Silent button above the volume keys is turned on (if off, orange sticker will appear). Then go to settings > sounds > Ringer and Alerts > change the volume to loud. The vibrate options can stay on. If all of these settings are enabled, try changing the ringtone.
For additional assistance, go to http://www.apple.com/support/iphone/