Why Carl Icahn's Buying a Stake in Nuance

Shares of Nuance Communication closed almost 6 percent higher on Tuesday after activist investor Carl Icahn disclosed a 9.27 percent passive stake in the software company.

Most consumers may never have heard of Nuance, but its software has likely heard their voice. It's the leader in speech recognition technology, powering everything from Apple's Siri to banks' customer service calls.

Just Monday, Nuance announced a big move into voice activated ads that talk back. The company is partnering with ad agencies, ad networks and publishers to tackle the $8 billion-plus mobile ad market, with the goal of making them more engaging and effective.

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Carl Icahn

So what's the appeal for Icahn?

With a $6.7 billion dollar market cap, Nuance shares have declined over 20 percent in the past year, as its margins have declined on higher investments in R&D and marketing.

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In addition to Siri, Nuance also makes Dragon speech-to-text software, which offers transcription tools for healthcare providers, businesses, and the likes of Microsoft windows. And this month the company even announced the integration of Dragon Voice into Panasonic Smart TVs.

The question now: What does Icahn have planned?

The investment is passive, for now, but investors are speculating about how quickly he could turn active.

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Stifel issued an analyst note saying that with a debt-heavy balance sheet and a growth strategy that relies on M&A, it's no surprise that investors might be interested in looking at alternatives for the company.

Analyst Tom Roderick said he thinks Icahn is weighing four options: 1) lobbying for changes in corporate structure, like changes to executive compensation or a reduction in M&A activity, 2) waging a proxy battle for control of the board and the management team, 3) pushing to maximize value by divesting business segments, and 4) evaluating the potential for an outright sale of the company, which Roderick ssaid is less likely given the how disparate the business lines are.

—By CNBC's Julia Boorstin; Follow her on Twitter: @JBoorstin