N. V. Tatsenko introduction to theoretical phonetics of english

Has he done it? [h^z hi dAn it]
[haz hi dAn it]
[az i dAn it]
[zi dAn it].

5. 
   Stylistic Modifications of Sounds
   

Stylistic oppositions have long been observed in linguistic literature in the two marginal types of pronunciation: formal and informal. Formal speech suggests dispassionate information on the part of the speaker. It is characterized by careful articulation and relatively slow speed. Informal speech implies everyday conversation. The following definitions are also used: rapid colloquial speech, conversational style.
Now let us turn to different forms of communication. A monologue often presupposes public speaking with a considerable distance of the addresser (the speaker) from the addressee (the listener) or a piece of calm narrative. Dialogues are more often private, personal and intimate. Monologuing is characterized by more phonetic precision. On the other hand, speech may vary in numerous ways. The interaction of the extralinguistic factors may arrange the opposite situation: the speaker’s highly excited narration of some critical situation will become full of slurring while a dialogic discussion of problems between colleagues will be phonetically most precise.
Stylistic sound variations seem to have the tendency towards the increase of the sound modifications in speech with the quickening of its tempo and the weakening of the carefulness, e. g. government ['gAvanmant ^ 'gAvamnt ^ 'gAvmnt ^' gAbmnt].
It is assumed that in formal situations the participants will monitor their linguistic behaviour. If the speaker wants to be clearly understood (like while producing a lecture with an educational aim), he/she should sound explicit and the pronunciation may be characterized as supercorrect. In informal situations, where speakers are more relaxed, less attention will be given to speech and more natural and simplified it will sound. Consequently, the degree of simplification of speech (assimilation, reduction, elision) may be looked upon as a style forming means.
Typical character of sound simplifications in relation to the degree of formality is the great qualitative stability of vowels in slow formal speech and more frequent sound variability in formal spoken English. Both front and back vowels in less explicit articulation tend to be changing towards neutralized sounds, especially in grammatical words.
S
Formal
its 'nut bi'koz
ai'Giqk hi: waz


Informal
3ts 'not bikaz
л'Шдк i wz [i:] tends to lose

pelling
it's not because I think he was
T
its

he historically long vowel diphthongization; as the next stage it undergoes quantitative reduction and finally changes its quality as well.
Spelling Formal Informal
I don't believe it ai ' d3unt bi'li:v it л d3un(t) ba'liv it it seems to be it 'si:mz ta bi it 'simz ta bi The similar process of reduction is likewise observed in [u:] simplified to [u].