The MP3 music format is losing ground to rival technologies backed by Apple and Microsoft, according to researchers at the NPD Group’s MusicWatch Digital who track the contents of people’s hard drives. The researchers said that the percentage of MP3s in digital music collections has steadily declined in recent months, down to 72 percent of collections from 82 percent a year ago. The AAC (Advanced Audio Codec) format, which Apple’s iTunes Music Store uses, and Microsoft’s WMA (Windows Media Audio) format has each gained about 5 percent of the “hard-drive share” in the past year.