The Rock Band franchise has sold millions, but according to MTV Games parent Viacom, the series fell short of expectations for the fourth quarter ended in December.

Viacom CFO Tom Dooley said in a Thursday earnings call that Rock Band 2 sold nearly 2 million units ("hardware, software and bundles") since its debut in September 2008 across all platforms.

But a challenging retail environment kept sales of the sequel below Viacom's projections, Dooley said.

"While we remain enthusiastic about the future franchise potential for Rock Band, the impact of a slowing economy dampened consumer purchases of videogames in the quarter, resulting in lower sales of Rock Band than we had originally hoped."

Viacom also said that the Rock Band franchise has shipped over 10 million units since its November 2007 debut, and ranked "as the #1 game title of 2008 by revenue across all genres" worldwide in 2008.

Dooley also said Rock Band 2 sales drove international ancillary revenues, which were flat at $462 million. The sequel also partially offset an 8 percent decline in domestic ancillary revenues.

MTV Games-owned Rock Band developer Harmonix is currently developing a Beatles-licensed music game.

Overall, Viacom reported a 69 percent year-on-year drop in net earnings to $173 million for Q4, due to $454 million in restructuring charges. Quarterly revenues were $4.2 billion, or flat year-on-year.