This is not always under concious control.
I hate myself for it, but I've started singing "da" instead of "the" in Irish-y songs. And I know its just because an Irish bloke I talk to down the pub has a perfectly genuine tendency to do that in his speech.
I don't know in advance that I'm going to do it - and then it grates like hell when it comes out!
Ditto some faux-northern bits in the trad English songs - due to listening to too much Waterson/Carthy.