I'm an English teacher (English as a Foreign Language)
so I have to use the IPA all the time. In connected speech, as in sentence level, uninterrupted speech, '[should] have' and '[should] of' do sound exactly the same (/əv/) . The only time we hear the pronunciation /hæv/ is when we are using the word in isolation as a standard verb - not when it's an auxiliary. MAYBE it's used to denote stress of that part of the phrase in a conditional. I don't think this is likely though, we're more likely to stress 'He SHOULD have come', 'HE should have come' or 'He should have COME' than 'He should HAVE come'. This is because as 'have' has a grammatical rather than standard verb function here we don't have reason to stress it.

All the textbooks I use explicitly teach students TO pronounce 'have' in perfect tenses in the way you say is 'incorrect' (/əv/). Frankly, it isn't incorrect. It's how we use the language, I'm afraid.

/end grammar rant
(Turn that bloody gramaphone down has incorrect spelling in her username *WINS*, Sun 3 Apr 2011, 18:18, 1 reply)

Oh yeah...
and 'their' and 'there' (and I would say 'they're', too) are pronounced /ðeə/ in RP. I wouldn't try to use the IPA unless you actually know how to use it properly...
(Turn that bloody gramaphone down has incorrect spelling in her username *WINS*, Sun 3 Apr 2011, 19:18, closed)