Music In My Heart (1940)
In Music In My Heart the girls sang part of Punchinello, the song to the accordion-player's monkey. This starts off being sung by Tony Martin. He's very competent, but the start of the song is rather unattractive in its pseudo Italian opera style.
The band then strikes up with a swinging rhythm and The Brian Sisters bring life into it.
"Oh Punchy, oh Nello" may not be strictly correct, but it is good fun! They get it just right. They pass phrases between themselves expertly & effortlessly, and continue with Gwen (only twelve but looking very grown-up, sitting in order to increase her apparent height) singing “we're inviting you to sing it with us” to Tony Martin… which he does. Much more Punchy-Nello when the girls appear! They clearly enjoyed this performance.
Unusually, Betty (rather than Gwen) takes the melody at the beginning. As the song progresses they revert to the usual arrangement.
This time, though, they don't appear in the credits.
This film is readily available on DVD.