VoodooVox, which runs an ad network that places audio ads for radio stations, calling card and 411 service companies, is turning its attention to the emerging – but yet to take off – Web-embedded telephony market.

While seemingly dozens of companies have launched widgets and sites aimed at enabling voice communications over the Web, few have taken off, and even fewer have demonstrated they’ll be able to generate revenue.

Web click-to-call vendor Jangl recently turned on audio and text ads, in partnership with Pudding Media, another vendor hoping to monetize Web calls with audio ads. Another player, Jaxtr, recently came out of beta with what it claims are more than 10 million widget downloads but little in the way of revenue. It launched Café Jaxtr, a voice-based social network that will carry advertising.

VoodooVox is taking a different approach. This week it released the MyVox API, which lets Web sites integrate audio from site visitors into their applications. Users call into a site using a regular phone and leave a message, which is then recorded and automatically mashed up into the Web site/application. Audio ads are delivered during the dial-in phone call but not on the Web site, so the site user experience is not altered or delayed.

The service aims to build on the success of VoodoVox’s off-line business, which today inserts ads on more than 250 million calls per month for more than 500 publishers. For instance, radio stations use the service to encourage listeners to dial in and leave voice messages for DJs; VoodoVox delivers an ad when the caller hits the message line.

VoodooVox is taking its business model – a la Google AdSense and DoubleClick’s ad network -- and even the terminology it uses from the Web advertising world. For instance, it calls the companies it works with “publishers” and the calls they drive “impressions.”

Embedding communications in Web sites and applications – rather than just focusing on free or low-cost Web-based calling – will result in more creative, widely-used Web voice apps, said J. Scott Hamilton, President and CEO of VoodooVox.

“It’s a very exciting time, and when we look back in the rear-view mirror, we’ll see that a lot of new, novel phone applications are being created,” said Hamilton. “It’s just that I don’t think a lot of these first-generation applications are the right ones. That said, there’s something definitely interesting about adding voice to the mix at the Web application layer.”

VoodooVox’s proposition is that any developer can access its MyVox APIs to add voice to any site or application they’d like. VoodooVox handles the voice back-end – using the same infrastructure that handles its landline business – and shares revenues with publishers with a 50/50 split. Typical audio ad fees will carry at $20 CPM (cost per thousand impressions), Hamilton said.

According to Hamilton, Web-based person-to-person calling certainly has its place, but may not be the place for audio ads, as evidenced by early user reluctance to listen to pre-roll audio ads on Web telephony calls.

Rather, Hamilton points to information services like MovieFone, 900 joke lines and 411-style information services as being a better model for driving audio ads on the Web.

To show off the audio-enabled mashup apps, VoodooVox has released its MyVox APIs and created a sample application gallery. The sample apps include voice notes added to a Google map; a voiceover narration of a Flickr slideshow; and friend comments left on a Facebook page.

Hamilton pointed to VoodooVox customer Web service Blabberize – which lets visitors add voice annotations to pictures you upload – as the type of fun, viral service that could drive millions of audio ad impressions.

“What we’re saying to developers is: If you’re building a Web site or widget, give it a voice,” Hamilton said.

VoodooVox’s “stretch” goal is to serve 1 billion voice impressions per month by the end of this year, the key being that no one service can hit that number, but a network of Web voice services just might, Hamilton said.

VoodooVox is backed by venture capital investors including Apax Partners, Steamboat Ventures (The Walt Disney Company), Softbank Capital and Village Ventures.