Market research firm specialist NPD Group has revealed that Apple has held onto the US top spot for music sales in the first 6 months of the year.

The NPD data is based on stats from respondents aged 13 and up, who reported on their purchases of physical CDs, digital music, as well as over-the-air wireless transactions, but not subscription-based online services.

According to the "MusicWatch" survey, Apple is in the lead, with American retailers Wal-Mart and Best Buy following in second and third.

Online retailer Amazon moved up to fourth on the list, thanks to the launch of its Amazon.mp3 online store that offers DRM-free MP3 downloads.

Despite Amazon's move up the charts, NPD sees Apple keeping its lead: "We expect Apple will consolidate its lead in the retail music market, as CD sales continue to slow", said Russ Crupnick, entertainment industry analyst for NPD.

"Amazon’s CD buyers tend to be older, so they haven’t abandoned the CD format to the extent seen in the average music buyer."