Accessibility certification or Audit

I do not mean to cast aspersions on your idea, but have you done research on whether such a tool is necessary? If you have not already done so, I suggest you research screen readers and how they work. Kudos on wanting to make sure your tool is accessible, though.

I'm not sure what you mean by a "voice interface". Perhaps you are thinking about making an interface whereby the webpage can read aloud text, or maybe you're thinking about a method for allowing users with disabilities to navigate through the website using voice commands.

Both approaches are not really useful to a substantial audience. People with disabilities use a variety of assistive technologies including screen readers which read aloud the text of webpages to people who are blind and have keystrokes or touch-screen gestures that allow blind people to navigate webpages; screen magnification software that makes the text of webpages larger and more easily visible for people with low vision; software that allows users with dexterity issues to navigate webpages by using voice commands; and alternative input devices for people with dexterity issues.

As a web developer, all you need to do is to ensure that your website complies with the <a href="https://www.w3.org/WAI/standards-guidelines/wcag/">Web Content Accessibility Guidelines</a>, the standard that guarantees that your website will be usable by the vast majority of people with disabilities.

so how do you know where your website stands? A free tool to evaluate the accessibility of your website is <a href="http://wave.webaim.org">WAVE</a&gt;. Note that WAVE can only check for a very small set of accessibility issues.

You also speak about an audit. Web accessibility audits can be very expensive and time-consuming, and the report you receive may not actually be useful unless you know a lot about web accessibility. For small companies, the approach that is far better is to learn about web accessibility through websites such as <a href="www.webaim.org">WebAim</a&gt; and to make your website accessible based on what you have learned (probably to WCAG conformance level A or AA); and then to put your website's accessibility to the test by asking users with disabilities to test the functions of your website using their preferred assistive technology. Websites need to be usable as well as accessible, i.e. just because you have met all of the success criteria for Level AA does not mean that people with disabilities will be able to efficiently use your website. If you want to make a truly accessible website, do not neglect to encourage people with various disabilities to test your website.

Once genuine users with disabilities know your website is accessible, you can then declare it to be "certified" as accessible. Announce it on the website and spread the word.