I think that the difference comes from Dynamic Range Compression in the audio tracks (
http://en.wikipedia.org/wiki/Audiolevelcompression). I made one ringtone out of an old classical recording, and it is way too quiet. I made another one out of a more recent rock song, and it was plenty loud.
I also think that the little built in speakers are better at playing certain frequencies than others. For example, songs with a lot of low end are going to sound very quiet because those portions are just getting left out.