Are you guys just direct linking to the offer page straight from the ppc ads, or are you creating a custom/buffer page? I've never done ringtones before, so this is just an idea, but I would think that if someone has seen the other ringtone pages, and came to yours and saw something different they may be more inclined to fill it out.
Also, if you made your own page, you could potentially highlight lists of songs/tones that they may be interested in, or even better, get some php on there to list the keyword onto your site that they just came through searching for, and maybe that will increase successful cpa's?
I've also heard from very good sources that many times you do actually get more signups than are recorded, but they aren't counted because they are dupes. Try switching offers, it may not pay an insane amount, but because they don't have a big list of members you can probably get a lot less dupes and more successful transactions.
Just an idea.