Shareholders of Audible.com are making noise about the company's proposed $300 million sale to Amazon.com.

Red Oak Partners, a New York City hedge fund that owns 1.4 percent of Audible shares, denounced the proposed $11.50 per share sale in a letter dated March 6 to the chief executive of Audible, calling the terms "inadequate" and "below fair value."

The critique from portfolio manager David Sandberg follows the filing of a class-action lawsuit February 20 in the Superior Court of New Jersey that charged six of Audible's directors with breaching their fiduciary duties with the aid of Amazon.

In trading Friday afternoon, shares of Amazon.com gained $.18, or .3 percent, to $62.92, while Audible edged up $.01, or .1 percent, to $11.47.

In the letter addressed to Audible Chief Executive Donald Katz, Mr. Sandberg called a valuation analysis by investment bank Allen & Company "flawed."

From March through July 2007, Allen & Company offered Audible to a dozen potential suitors for $12.50 per share. In a later analysis, however, Allen trimmed its assessment of Audible's worth to $11.50. Amazon, which last year introduced its Kindle digital book reader, announced the Audible acquisition in January.

Mr. Sandberg said Allen & Company trimmed Audible's valuation "despite a year in which Audible grew its revenues by 34 percent and added $0.50 per share in cash to its balance sheet.

The Red Oak letter also questioned the $2.62 million fee charged by Allen & Company.

Audible derives more than 20 percent of revenue from audiobooks and other content sold through Apple's iTunes online store. Its contract with iTunes runs through 2010.

Audible reported fourth quarter net income of $4 million, or $0.16 per share, on net revenue of $31.1 million. That compared to a net loss of about $900,000, or $.04 per share, on revenue of $23.2 million in the prior year's quarter.