This is the "A Choir of Angels: From Left Hand Shutter" painted in the Flemish tradition by Simon Marmion around 1459. The painting depicts a beautiful scene of three angels playing shawms, a precursor to the oboe, within an elaborate Gothic canopy.
Look closely at the bottom of the panel and you'll see the outline of a Gothic building - this is actually the interior of the Abbey of St Bertin in northern France, where the altarpiece that this painting once adorned would have sat.
The two lower angels hold scrolls, but these aren't actually musical notation - they were meant to be symbols or perhaps even decorative elements. The artist was experimenting with design and composition as he worked on this piece, changing details like the angels' instruments, heads, and wings between the underdrawing stage and the final version.
You can see the original unpainted edges along the tops and sides of the panel, but the lower edges have been modified - in 1822/3 a French dealer removed parts of this and its companion piece to fit them into a more conventional shape for a picture gallery.
A Choir of Angels: From Left Hand Shutter hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.