João Gilberto
 

I know that João Gilberto is Portuguese but I was wondering about the accented letter a. Is this unique to Portuguese if not how is it pronounced?

TIA

Desmond.

Yes, the A in Portuguese, at least Portuguese spoken in Brazil, is soft. It sounds like the A spoken in classic metropolitan New York City English.

The vowel 'ã' is not unique to Portuguese. This letter is used in Portuguese, Guaraní, Kashubian, and Taa, with basically the same nasal 'a' sound.
Other languages that use the letter, like Greenlandic, Vietnamese, and Aromanian, do not pronounce it the same way.
The letter 'ã' also appears in the International Phonetic Alphabet, and represents the nasal 'a' sound of the Portuguese language.

There are several ideas and videos online that will help you to pronounce this Portuguese vowel.

What do you mean when you say "accented letter a"?

If you're referring to the letter "ã" as in the name "João", in Portuguese the "~" marks a nasal diphthong. The nasal diphthongs that are spelling using "~" are "ão", "ãe" and "õe".