Lavkas & Tsimpidis -- Don't make me a man / Into the water of the lake

 Life with the swells...

Hopefully I have this right.  There's a shindig with an upper-crustish crowd..  A pianist and violinist are playing but not wowing the small soiree.  Fortunately, the bouzoukis show up in the nick of time.  The classical musicians are recruited (paid off?) into the band, a partygoer is recruited to sing (actually two), the party gets hotted up, and there is dancing and hi-jinks.

I've been to parties like this in the upscale neighborhoods of my town, with banjo and fiddle players, so I find this scenario 100% believable. 

From the 1958 film "Unlucky in Love". Here is a quick translation of the plot summary from greek-movies.com.   A janitor, Michel, a florist, Grigoris, and a taxi driver, Aristotle, lose their jobs on the same day and on a bench, where the unlucky take refuge, they get to know each other and become friends. While they are walking, they find a letter on the road from which they are informed that a very wealthy Greek-American, John Hadjikumar, is coming to Greece to meet his nephew Mike, whom he has seen since he was a child. Penniless as they are, they drop the big idea and plan to present Grigoris as Mike. After several ridiculous episodes, order is restored as usual in these films and everything goes as it should.  According to Yiorgos Vavoules Βαβουλές in the comments,  Hatzopoulou's voice is heard while Lydia Stefanidou appears to be singing in the film