I first encountered the expression "a finger in the ear" at Mudcat, and it really puzzled me. (I was taking it too literally—but then, I had no other way to take it.) I posed the question back in January, 2002, here: Help: Why a finger in the ear?

British folkies are also fond of working the expression into parodies:

"The traditional singer's wife,
Was sighin', "Dear, oh dear!
He never performed a' his life,
Wi'out his finger in his ear."
--from THE FOLKSINGERS' BALL (parody of CHARLADIES' BALL), in the DT.

"I have sung them a capella with my finger in my ear
Finger in my ear
Just the way a real folk singer should…"
--from MUST I THEN (a parody of MUSS I DENN) posted by Alan of Oz, 1997

"I've been a folk singer, for many's the year,
I sang unaccompanied, with my finger in my ear,
But now I get bookings, and I want to get more,
So I never shall wear wooly jumpers no more."
--from WOOLY JUMPERS NO MORE (parody of WILD ROVER) posted by GUEST,JMCC, 2000.

"At a folk club in a toon, I heard a a young man singin',
Like a constipated donkey in distress.
An' wi' a finger in one ear, the noise he made was weird,
And he went from bad to worse wi' every verse."
--from THE MURDERED SONG (parody of FIELDS OF ATHENRY) by Andy McKean

"And when I sing traditional, I stick my finger in my ear
Because half the songs I sing, I just can't stand to hear—but I'm an artist!"
--from THE FOLKSINGER (parody of Paul Simon's THE BOXER) by Fred Wedlock