grammar - "a" or "an" ubiquitous?

I am unsure whether to use "a" or "an" in the following sentence:

Video games have become a/an ubiquitous part of American culture.

For me, saying the two sentences out loud makes "an" seem like the right choice but Microsoft Word proofing disagrees.

Answer

This isn't straightforward. In my version of English, I would say 'a ubiquitous'. It seems that the grammar checkers in MS Word agree -- both in US and UK English.

However, take a look at this ngram of published works.

Google ngram: a ubiquitous,an ubiquitous

You can see that a changeover occurred in the late 1880s but both versions survive up to the present day.