Sound Filters

Sound Filters is a small client-side mod that makes Minecraft audio more immersive: it adds reverb in caves and muffles sounds when you are underwater or when the source is behind a wall.

Files cover Forge on Minecraft 1.6.4 - 1.16.5, and the author allows it in modpacks. Since it works on the client, it fits best in the modpack your players use to join your Minecraft server, and the panel adds it in one click.