THREE COLORS BLUE came to mind instantly, too.
Also THE SILENT TOUCH by Krzysztof Zanussi with Max von Sydow and music by Kilar using his concert work “Exodus”.
Isn’t ESCAPE ME NEVER with music by Korngold also about a composer? Don’t remember anything or if I actually saw the movie though…
The Columbo episode MURDER WITH TOO MANY NOTES was about a film composer as a suspect (I thought there was another older one from the 70s but can’t remember/find it) 😉