Factual error: Whilst at Sandringham estate in 1936 Bertie is waiting for the Tiger Moth plane to land. This particular plane (G-ANFM) was not built until 1941. (00:38:50 - 00:39:40)
Factual error: In the scene of King George V's 1934 Christmas Broadcast from Sandringham, there are factual errors. The B.B.C. Engineers are shown operating Outside Broadcast equipment in a room adjacent to the King - but the equipment (BBC Type OBA/8) didn't even come into service until 1938. It should have been type OB/7 in large mahogany cases. Also, the ornate Royal microphone cases were the wrong ones. These were the later (c1937) type first used by King Edward VIII in his Broadcast in 1936.
Factual error: In the early scene where the BBC announcer is introducing the Duke of York, the "ON AIR" sign appears to be in Helvetica font. This font was was designed in 1957, over 30 years later.
Factual error: Prime Minister Stanley Baldwin is shown resigning due to the failure of the government's appeasement policy towards Adolf Hitler. Baldwin resigned in 1937 as he chose to retire, appeasement continued under his successor, Neville Chamberlain until 1939 when war was finally declared.
Factual error: The Duchess of York's hat at Balmoral was not her fashion until the war years, (based on the Royal Air force themes).
Factual error: In the conversation with Bertie after delivering the Christmas proclamation, King George V refers to Joseph Vissarionovich Stalin as "Marshal Stalin". In reality, Stalin was awarded the title of "Marshal of the Soviet Union" in March 1943 after the conclusion of the Battle of Stalingrad and more than 7 years after the death of George V.
Factual error: A Humber Pullman, reg no UJ7 847 is in the scene where the Duke of York enters No 10 Downing Street via the garden entrance. The scene in which it appeared was before Edward VII's abdication, but the Humber was first registered 30 Dec 1936, which is after his abdication.
Answer: The technique is called "masking" and all though it can sometimes help those who stutter it is not 100% reliable. It will make some stutter less, others will not notice any difference. It would probably be very inconvenient to Bertie not to hear himself at all during an important speech, not to mention that those around him might find the whole thing strange. You can read more about research on this technique and other similar ones here: http://ajslp.asha.org/cgi/content/abstract/16/1/77.
littlestar