If the 'mispronunciation' is consistent, then it is an accent. The vowel combo 'au' is rendered as a short O in American English very consistently.
Ostralia, Ostria, Clod/Clodette, clostrophobia...
They even teach it in schools:
www.theschoolhouse.us/lessons/lesson49.html
The letters (aw, au) have the same sound as short (o).
The word 'clod' would be pronounced differently in American English than it is in British English. It has more of an A sound.
There is a subtle difference between Dawn and Don in American standard English.
Not many Americans are called upon to pronounce Hereford in the course of their lives. If they attempted it, they would most likely introduce a very subtle glottal stop after 'her'.
They would use the rhotic R too for the last R, unless they were in Boston or New England in general, which would be very noticeable to someone speaking British English. Rhotic R is not 'wrong'. It is a perfectly valid pronunciation in all cases.