The Om sound is obviously not just "mmm" sound. Otherwise it wouldn't be written as "Om", would it?
And your confusion between "Aum" and "Om" is not really warranted. It's just Sanskrit sandhi.
A + U = O
So the separate sounds A, U and M combine to form OM.
Mandukya Upanishad 1:
ओमित्येकाक्षरमिदं सर्वं तस्योपव्याख्यानं भूतं भवद्भविष्यदिति सर्वमोङ्कार एव । यच्चान्यत्त्रिकालातीतं तदप्योङ्कार एव ।
The single syllable Om is all this. Its explanation is that it is the past, present and future, all that is Om. Whatever is beyond the three phases of time, that is also Om.
Mandukya Upanishad 8:
सोऽयमात्माध्यक्षरमोङ्कारोऽधिमात्रं पादा मात्रा मात्राश्च पादा अकार उकारो मकार इति ।
This Atman is explained in regards to syllables as Om. In regards to the individual sounds (mātrā), the sounds correspond to the modes or phases of Atman. The sounds are A, U and M.
Mandukya Upanishad 9, 10, 11, 12:
जागरितस्थानो वैश्वानरोऽकारः प्रथमा मात्रा...
The waking state named "Vaishvanara" is the first sound 'A'
स्वप्नस्थानस्तैजस उकारः द्वितीया मात्रा ...
The dreaming state named "Taijasa" is the second sound 'U'
सुषुप्तस्थानः प्राज्ञो मकारस्तृतीया मात्रा ...
The deep sleep state named "Prājña" is the third sound 'M'
The fourth is without any sound because it is not expressible.