The best alternatives to OpenHub are CSS Galore, Soundy, and Nature sound. If these 3 options don't work for you, we've listed a few more alternatives below.
Soundescape is a free web application for ambient background sounds. If you struggle to concentrate while working from home, this tool is for you! Audio is rendered in 3D, and you'll never hear the exact same combination of audio twice, as everything is generated randomly.