In the two months since MTV Networks and Harmonix released the music-based video game “Rock Band,” players have purchased and downloaded more than 2.5 million additional songs made available after the game’s initial distribution.

Activision, meanwhile, said it has sold more than 5 million new songs via download for “Guitar Hero III: Legends of Rock” since it began adding downloadable content in early November.

By comparison, it took wireless operator Sprint four months to sell 1 million songs on its over-the-air full-song download service. While new digital music services competing with Apple’s iTunes and free peer-to-peer services have struggled to convince music fans to pay $1 for a single, downloadable tracks for games like “Rock Band” and “Guitar Hero” are flying off the digital shelves.

“We live in a rough time around music where our audience struggles to pay $20 for a CD but don’t hesitate to pay $50 for a game,” MTVN Music Group/Logo/Films division president Van Toffler told Reuters news service. “The notion to pay 99 cents or $1.99 to have a song and repeatedly play with it apparently isn’t a big hurdle.”

The original “Rock Band” and “Guitar Hero” games shipped with more than 50 licensed songs each, a mix of master recordings and covers. Since then, “Rock Band” has made new music available every week as either singles or in three-pack bundles that can be added as new playable levels for between 99 cents and $5.50. “Guitar Hero III” did the same, focusing on three-song bundles of new music and music featured in previous versions of the game.

Although MTV is not providing specific numbers, it did say that the majority of the downloaded songs were purchased by Xbox 360 as opposed to PlayStation 3 users. “Rock Band” sold 775,000 copies for the Xbox 360 through the end of 2007, compared with 250,000 on the PS3, according to market researcher NPD Group.

Whether bands are benefitting unilaterally from the sales bump isn’t clear. MTV declined to offer details but said that the Metallica three-pack of “Ride the Lightning,” “Blackened” and “And Justice for All” is the best-selling “Rock Band” download.

According to music monitor Nielsen SoundScan, those same songs enjoyed digital download sales increases of 31 percent, 39 percent and 48 percent, respectively, for the month after they were featured as a “Rock Band” download, over the previous month.

But those increased sales numbered only in the hundreds, while the “Rock Band” downloads numbered in the hundreds of thousands.

Music executives are impressed with the results regardless. MTVN has plans to expand its outreach to artists, creating additional game expansions — as both physical products and downloadable content — around specific music genres and even artists.

With this level of interest stirring, other games likely will begin offering new downloadable soundtracks as well.

Titles like the “Madden” football series, the Tony Hawk skateboarding franchise and the venerable “Grand Theft Auto” games are well known for their extensive soundtracks. And offering gamers the ability to replace their soundtracks every few months after the initial release is technically possible.

Editor’s note: Reuters, The Associated Press and IDG news services were the main sources for this report