I agree that this phrase seems slightly odd in this context. But the greeting is strung out into a dialogue which is itself rather artificial. The logical reply would be 'Thank you". Perhaps the "that's OK" implies that Mr Brown is a great man, used to being respectfully addressed. Though here there are no other words, this is true in the English words in a 1933 film version, probably based on a German or Austrian original.
Every day in every way, we must obey
Our Mr Brown from London town
What he does and what he says, must be quite OK
To please him, there's only one way
We must say Yes to Mr Brown
No more no less to Mr Brown
He is the great white chief you know
And so no-one must say him No
Наверняка трудно поверить, но в оригинале, столь популярное в тридцатые годы произведение, было написано как венский вальс... к фильму "Zwei Glücliche Herzen". Вальс назывался "So küßt man nur Wien". Кто в последствии трансформировал его в фокстрот, сам ли Пауль Абрахам, или это было сделано без его ведома - непонятно, но всеобщюю известность произведение получило благодаря именно этим действиям.
This seems to bee an illigal dubbing of Odeon Be 9850 (order no O-11639), recorded 23th of march 1932 in Berlin by Bravour-Tanz-Orchester Hans Bund with an unnamed choir, that sounds just like a group called “Sing-Sang Artists”, whose rendition is on Parlophon 128295 (later changed to the cheaper Gloria Bi 641).
“How do you do Mister Brown” uses the same melody as “So küßt man nur in Wien”, as both are sung in the film musical “Ein bißchen Liebe für dich” (subtitel: “Zwei glückliche Herzen”) made by the H. M. company. Premiere took place in Berlin, 3th of march 1932 (according to other sources one day earlier in Wien as “Geschäft mit Amerika”).
Simultaneously a French version “Monsieur, Madame et Bibi” was made and premiered 12th of march the same year. Shortly afterwards two remakes appeared: “Due cuori felice” in Italy and “Yes, Mr. Brown” in Britain.
The original movie is no masterpiece and perhaps only worth watching because of its musical numbers. The female lead is played and sung by Magda Schneider (today mainly remembered as mother of Romy Schneider), but in both respects exceeded by the now forgotten Lee Parry.
“How do you do Mister Brown” is carolled succeedingly by the staff of a viennese automobile shop during their boss’s arrival. Mr Brown is known to have never been in Europe before. The song doesn’t consist of its refrain only as the above record may suggest, but nevertheless doesn’t have to be taken too seriously.