Ummmmmm
Just in from one of my esteemed elderly relatives (born NSW, Australia 1910) ........
"Erm" used in NSW public service workplace conversations until mid 1950s when it was abandoned in favour of a slight cough immediately followed by "Ahemm" and then the speaker went straight into verbal commentary without regard to the conversation being interrupted.
"Erm" was spoken quietly, whereas "Ahemm" was spoken aggressively, and if speakers were all standing, the speaker would take a step forward.
If seated (usually in a formal meeting) and allowed to continue speaking, then the minute taker may have included the significant words in any official records, but if an "Ahemm" user was 'put back in their place' by the chairman then the minute taker would usually note "Chairman reminded all present to refrain from interjecting".
I then asked about "Agh" and I am informed (by esteemed rellie) that it is a long standing expression, used frequently throughout his life, so its NSW usage may begin possibly back into early 19th century. It is simply a polite way of saying "Phew, so you've finally understood and I don't have to keep repeating myself"
I also asked about "Um" and I am informed that "Yes, of course, those RChatters are right, it is an interjection, a polite one, seeking gentle quietness for those present to actually pause, look at the speaker, listen and comprehend, so that they can contradict or agree with the speaker."
And errrrrrrrrrr .... then some people wonder how I can be so long winded in my own explanations...
JM