RealNetworks Inc. will lay off part of its work force this afternoon, according to sources close to the company. The number to be laid off and their positions aren't known.Martine Charles, RealNetworks' vice president of corporate communications, declined to comment.Some in the industry speculated that the company's recently announced foray into open-source software is behind today's layoffs. But the economy is probably more to blame, analysts said.RealNetworks last month announced Helix, an open-source set of software products it said will allow creating digital media files and programs for any format, operating system or device. In theory, the company's move toward...