If you're trying to boost the built in speakers for it, here's what I did when I owned one.
Go to your System Sounds by hitting the Windows Key and typing "Change System Sounds" and running that. A small window will show up with 4 tabs which are "Playback - Recording - Sounds - Communications", under the Playback tab, look for your Rift S and go to it's properties. There you will see a tab that says "Enhancements", there you should find a "Loudness Equalization" Check box, check it and apply, this will boost the volume noticeably, but keep in mind that if you plan to plug in any headset to the Aux port, disable this if you notice weird changes in volumes.
Whenever there's a big bang of a sound, this tries to lower it I've noticed, but for the most part, it gets the job done. Hope it helps.