Jane Birkin...
Audrey Hepburn
I'll second Eliza Doolittle.
I'll second Eliza Doolittle.
Can't believe I had to read through everything to check only to find the last post seconded AH. (Assuming of course you are referring to the character rather than the current 22 year old singer/songwriter. If you are then I second Audrey).
Audrey Hepburn was lovely. But this is about musicians and singers.
She couldn't and didn't sing so I really don't see how she qualifies.
She may not have sung in My Fair Lady but I would respectfully refer you to her rendition of Moonriver in Breakfast at Tiffanys.