Replying to Mobile 17 Ringtone Creater Issue

I've been messing with this site http://mobile17.com/ and have made a few custom ringtones, but the problem that i'm having is that some sound good and the quality of others are shit. the first one i made is 10 seconds long and sounds great...then i made one 30 seconds, sounded like shit and figured it was because of the length.  so i made another one at 20 seconds and that one sounds good....after that i tried making more that were even shorter (like 15 seconds) and those sounded like shit...i used a wav editor to trim the files but i don't remember what i exported the good one at...any one know how to get them sounding good all the time?