I don't know if Ringtones.net is still being run by Viveli (Eduardo) but they have always been extremely helpful and easy to work with whenever problems have arisen in the past.
I can't speak for CPA Empire as a gateway to the offer, but I can tell you that the team behind Ringtones.net is one of the most honest + professional in this industry - Grade "A" in my opinion - and absolutely would not shave leads to compensate for higher payouts.
Depending on what sort of past conversion/metrics data you have, it may be worth compiling some reports and sending it their way to try and determine what may be the reason for the sudden drop.
That said... depending on what sort of traffic you're sending through, it is plausible that they won't buy shit traffic if you're not sending premium carriers as well. These companies actually lose money often times on second/third tier carriers but make it up with premium carriers, so make sure you're sending quality traffic.
One thing I'm noticing now (for those still promoting tones) is a lot of people sending premium carriers to Net Partner (funmobile) and sending crap traffic to ringtones.net. I wouldn't buy that traffic either if it was a losing preposition.