Rapper 50 Cent may have sold more CDs than anyone else last year, but he's now vying for another crown: Lord of the Ring Tones. With the music industry desperate for new avenues to boost its bottom line, labels and artists have made digital content for mobile devices -- that would be those robotic cell-phone ditties -- a huge priority this year. At $2 a pop, ring tones account for upwards of $2 billion in revenues worldwide. As the global latecomer to this party, the U.S. makes up only a sliver of that action. But with domestic mobile technology advancing by the nanosecond, Stateside revenues (estimated to hit $100 million this year) are poised to explode in the next 12 to 18 months as tech-savvy consumers upgrade their gear. ''It's definitely the fastest-growing segment of the digital music market,'' says the aptly named David Ring, VP of business development at Universal Music Group/ eLabs. ''In some cases, we've seen [ring tones] outsell their digital download counterparts.'' While Universal's top sellers have mostly been hit singles from youth-skewing current acts like Hoobastank and 50, Ring notes that as the market expands, even aging stars will cash in. Never one to let an emerging trend pass her by, last week Madonna became the first artist to launch her own Web-based store hawking mobile goodies. Who knew a Muzak-y rendition of ''Lucky Star'' could signal the future of the record biz?