I never did find an equivalent for lock and unlock. Instead I just removed those lines and continued to use the code.
It was working fine until 5.4, which caused the audio to stutter. Seems like the first chunk of the file plays, then instead of moving on to the next chunk, it restarts, resulting in a stutter effect.
I narrowed down the issue to this specific commit: https://github.com/EpicGames/UnrealEngine/commit/cee22af214e58ce2c37854b611dcabac2b33640c
The function now checks if DecompressionState == nullptr and returns if so.
My current solution is to remove this check and recompile the source code. I’d rather use the binary version of the engine, so if anyone has a better solution I’d love to hear it.