Hollywood dialect coach Erik Singer -- who helped Austin master his Elvis Presley voice for Baz Luhrmann's flick about the King of Rock 'n' Roll a couple years back -- has come to the defense of the British actress ... telling TMZ people need to relax, 'cause MBB's 100% right.

Singer says ... "I really wish people would lay off of actors, and most especially that they'd lay off of criticizing people's accents."

He goes on to tell us it's entirely normal for people's accents to change depending on who they're talking to -- and he would know, since this is literally his job. ES says this phenomenon is called "accommodation" or "conversation" convergence.

In his experience, Singer says just about everybody does this ... especially actors.

Singer goes on to piece some things together about Millie's backstory and history -- including having grown up in both the U.K. and the U.S. -- and says that it's completely understandable that she might have a dual accent, and that different flavors come out in different times. In fact, he suggests Millie might be bi-dialectal ... which is a thing!

Short story long ... Singer says there's nothing surprising about MBB's accent flip-flopping.

Singer finishes by saying ... "If there's anything odd or wrong it's the fuss around it, and the insinuation that there could be anything wrong or odd about it!"

Millie did a pretty good job of defending herself last week -- saying she's living in NYC with her fiance, Jake Bongiovi, and that her accent changing based on where she's at/who she's with is something that happens all the time. Singer, it seems, would agree.

Erik's fierce defense of Millie isn't entirely shocking ... as his client Austin received backlash after he struggled to shake his Elvis impression.