The last major composition that Beethoven worked on before his death was an intended string quintet in C. He only managed to fully score the introduction to the first movement, but left behind substantial sketch and draft material for all four planned movements when he died in March of 1827. Even that beginning of a score is lost today, and exists only in a piano transcription by Anton Diabelli. What the quintet might have sounded like has captivated Beethoven lovers and fans for generations.
Composer Adrian Gagiu has produced a new realization of the quintet, working closely from Beethoven’s sketches. He has kindly provided mp3s of his realization to the Unheard Beethoven, where you may hear one conjecture for where the quintet might have gone, had Beethoven survived to finish it.
Gagiu’s realization can be heard at https://unheardbeethoven.org/search.php?Identifier=gagi62
A previous realization of the sketches by composer Hideaki Shichida can also be found here on the Unheard Beethoven site: https://unheardbeethoven.org/search.php?Identifier=shic62