Before anyone asks, yes, this game is missing A LOT of sound effects during the sex scenes, but not because you have an issue on your end. That has been caused because of the author being negligent enough to not put the sound effects when needed, even though for some of them they are present, so I am not sure what's the fuzz with that.
If you want to add them, you'll have to edit manually the CommonEvents.json file, but beware of this being a really time-consuming process.

Contact me: https://discord.gg/fjZKJJUWw4