You've heard of BlackBerry thumb. Now there's iPod ear. As use of portable MP3 music players soars, concerns are emerging that the gadgets may contribute to hearing damage. Some doctors say they are seeing younger and younger patients with signs of noise-induced hearing loss that wouldn't typically emerge before middle age. And they are worried that the constant use of MP3 players, which blare music directly into the ears, may be partly to blame. Similar concerns were raised when the first generation of portable music players, including Sony Corp.'s Walkman, hit the market in the 1980s. But the latest portable...