"Veglia, o donna, questo fiore" was an aria from the Italian opera Rigoletto by Giuseppe Verdi from the 19th century.
The Doctor hummed the aria in sickbay while treating Fantome, a member of a species that was native to the area where the USS Voyager was trapped in 2377, known as the Void. Fantome seemed to relax when he heard the song so The Doctor decided to play the full orchestral version of the aria, this enabled The Doctor to finally treat Fantome's injuries.
Seven of Nine turned off the music, much to The Doctor's and Fantome's disgruntlement, when his enjoyment of music gave her an idea to communicate with Fantome through musical notes. (VOY: "The Void")