Vincent McNabb is correct. If you want evidence based on "credible and/or official sources" that this is the rule followed in formal English writing, here is my suggestion.
I ran Google searches on Google Books only, meaning the bulk of the search will be against professionally edited and published works, not random web sites. I searched only for phrases where the pronunciation of the acronym was relatively clear and consistent: for example, nobody pronounces "SCUBA" as "Ess Cee Yew Bee Ay" and nobody pronounces "FBI" as "Fibbi."
Here are the number of hits in the Google Books database for:
- "A SCUBA": 49,800
- "An SCUBA": 56
- "A FBI": 16,000
- "An FBI": 343,000
- "A NASA": 264,000
- "An NASA": 16,500
- "A RGB": 7,130
- "An RGB": 33,800
- "A UPC": 11,800
- "An UPC": 436
In each case, basing the article on the initial sound, rather than on the initial letter, is more common; in most cases substantially more common.
As a control, I also looked at two acronyms where both the initial sound and the initial letter are consonants.
- "A VPN": 50,100
- "An VPN": 960
- "A OCR": 9,380
- "An OCR": 1,870,000
Because "An VPN" and "A OCR" are incorrect based on any possible rule, we can conclude that the positive results are grammatical, OCR, or search engine errors. This suggests that the minority viewpoint on SCUBA, FBI, NASA, RGB and UPC are also smaller than they appear.
We can conclude that, based on evidence of usage among published documents digitized by Google Books, the preferred rule is to base the article on how the intended pronunciation of the acronym would be spelled phonetically.