Is the sacred Hindu syllable a diphthong or a single vowel sound? It is rendered in the Roman alphabet sometimes as "AUM" and sometimes as "OM". The standard calligraphic form in the devanagari script suggests that is the diphthong - "a" "u" nasalised and not "o" nasalised.When the sound is voiced, however, it is more often heard as the single nasalised syllable "om".
According to the traditional belief, only initiated trivarnas can utter OM. AUM is for female and sudras. Some tantrik scriptures give permission to chant OM to all, but not all the tantras.
Ref: a)Saktasadhana o Saktasahitya, Upendrakumar Das, RMIC, vol 1 b) old copies of Nitya karma paddhati--- dont know whether they have changed now.
In my personal experience, the "Om"s I have heard from Indian priests and chanters is almost monophthongal. They say that Sara Palin has monophthongal "o" (like in her "also"). It is written aum because in sanskrit Sandhi a+u becomes o (any textbook on Sanskrit Sandhi rules would show this.