The hardest part is probably manual memory management, and getting used to expressing solutions using iteration rather than recursion?
I don't like C++ (and have mixed feelings about Rust) because of how large and complex the entire language is. If you'd like a stopgap where you can still enjoy the functional style, and don't mind compromising on raw performance, Python is great. The operator syntax is also quite close to C, and you do get bindings to GStreamer in case you decide to take your audio project in new directions.