Ringtones are a cash cow for the music industry, but new research shows that the market's growth will flatten out over the next four years and eventually begin to decline.

According to iSuppli, U.S. ringtones sales should total $700 million this year, just 10% over last year. That's a far cry from the 34% growth rate experienced last year and the triple-digit growth of years prior.

Growth rates will then fall into the single digits through 2010, the company says, at which point revenues will begin to decline, potentially by as much as 21%.

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