Before you order your accessibility audit you should read this article of mine. I try to be objective and constructive and present the good and the bad, the strengths and the weaknesses of accessibility audits.
Website owners are responsible for use of third party widgets, plugins and more. Before using them they should check if they conform to WCAG, otherwise their site will not conform either. Checking for accessibility statement of the third party may reveal huge problems with their product’s future.
We all reach out to third party solutions and we like it when they claim they are accessible. But please don’t just believe them – check that they really are conforming. And when they update – check again.