Amazon.com and Wal-Mart have both quietly followed Apple’s lead and will now charge more for some top-selling tracks in their MP3 stores. Some songs will now sell for $1.29 at Amazon’s MP3 store, up from 99 cents, while Wal-Mart is now charging $1.24 for top tracks, up from 94 cents. Apple introduced variable pricing to its iTunes music store Tuesday, by charging 69 cents for older tracks, 99 cents for recent songs and $1.29 for new hits, instead of the previous 99 cents for any track. Amazon’s decision to raise prices seems especially unusual, considering that the company actually cut… Read more »