Pronunciation of a/an.

[raindoctor]

There is a wide spread phenomenon of pre-velar nasal tensing (in general, pre-nasal tensing) in AmE: CAT vowel becoming BET vowel.

an apple: ɛn'æpəl (pre-nasal tensing)

[pizza]

The official version according to most sources is:
/ən; when stressed æn/

However, if you use Google dictionary to listen to the 'an' it sounds like an ɛ.