Kenney is well-known for creating high-quality reusable art assets, they've done a huge amount you can buy and quite a lot are also public domain under the CC0 1.0 Universal Public Domain Dedication license so you really can do anything.
They're also now doing audio assets! Giving developers in need something a little extra, on top of everything they already do. Just recently, they released the Interface Sounds pack which contains 100 public domain sound effects that anyone can download and use free (you can also donate). All high quality too—wonderful! Here's a preview of what to expect:
If you've not heard of them before, they also created Asset Forge. A cross-platform application to help you create models for use in games which is currently going through a huge overhaul.
Honestly, I think the work Kenney does is amazing. Just like we have people building open source games/applications, we also have people creating high quality art (and now audio) assets for others to use like this. Even the much bigger packs that Kenney sell, have a reasonable price tag on them and no license restrictions on them too.
See all their assets on the official site.