Play an audio clip every so often, with adjustable volume.
This very simple project was inspired by the Dungeons and Dragons adventure "The Lost Mine of Phandelver", which features a location called "Wave Echo Cave", so named for the periodic sounds of crashing waves echoing through its mysterious reaches. I use it to heighten the audio experience for my players without having to remember to push a button every couple of minutes.
This project contains an excerpt of "Waves on a quiet New Zealand beach" by RTB45, graciously made available under a Creative Commons Attribution license.