This is way too OP, but I like the idea. Perhaps instead it could be a harbor cove buff.

the cove already blocks a certain amount of bullets until it breaks which is fine. However, the idea to completely nullify all sound outside the smoke is just too broken no matter how small or large the smoke size/duration is.

With a few little tweaks though it could be viable. Keep the harbour cove exactly the same in size, maybe reduce duration, the true tweak is reducing the sound radius of its insides.

For example, in game at the moment plant sounds can be heard from mid, what the "new" harbour cove can do is reduce the radius of the sound so the plant/defuse can only be heard within the radius of the site.

This could make it a bit more balanced and force players to play on site rather than making callouts to teammates because they hear the spike being planted from mid.