By Prince McLean

Published: 10:00 PM EST (07:00 PM PST)

Apple has increased its lead in music sales in the US through iTunes, with 28 percent share of all music sold and a 70 percent of all digital music downloads.

Tied for second place is Walmart and Amazon, which both took a 12 percent share of the total market for both digital music and physical media sales. Amazon increased its music market share by 3 percentage points, while Apple's share grew by 4 points.

NPD says digital sales now account for 40 percent of the US market, an increase of five percentage points over last year. NPD also stated that Apple's share of the market for digital sales only grew by one percentage point to claim 70 percents share of downloads, while Amazon picked up 4 points to reach a 12 percent share of the digital market.

Russ Crupnick, vice president of industry analysis for NPD said "the decline in US CD sales means that selection and merchandising of the physical CDs is suffering, which is one of the primary reasons consumers say they purchased CDs less frequently.

"Online shopping offers consumers who still want CDs more variety than they would get in a brick-and-mortar store; plus, recommendations, and other interactive features that raise the overall value proposition for music buyers."

Despite Amazon's growth, Apple faces federal investigation from the Department of Justice to determine if the company is leveraging its market power to prevent competition in its negotiations with music labels.