So-called iPod killers are not just failing to compete with Apple’s popular media players, they’re failing to hold onto what market share thy have.

Sales of portable music players, besides the iPod, declined during the first three weeks of the holiday shopping season, according to the NPD Group.  Manufacturers face a market in which there are fewer first-time buyers, and most sales are for upgrades and replacements.

“The market is in a position where most of the people who want an MP3 player have one,” said NPD analyst Stephen Baker.