Free Sleep Sounds (free!) joins a pretty crowded field of apps designed to help you sleep, relax or meditate.
The app contains 25 well-recorded environments in stereo in categories like Ocean, Rivers and Streams, Wind, Fire, Birds and more. A unique feature is the ability to created blended mixes of any of up to six tracks, so you could hear wind along with the crickets. The app can run in the background.
Free Sleep Sounds also shows you some nice images taken around the world to accompany the sounds. There is a sleep timer, and the tracks nicely fade out so they are not jarring.
I tried Free Sleep Sounds with headphones as well as on a Bluetooth Stereo speaker, and found the audio quality quite good. Using this ad-supported app is easy enough, but it has the most intrusive and obnoxious ad placement I have seen in years. The screen bounces with notifications, and many of the ads are animated and distracting. It is the LAST thing I would want to see in an app designed to help me relax. If you look at the screen at all, anxiety replaces any soothing effects the soundscapes are designed to create. At times I wanted to throw my iPhone out the nearest window.
The same developer offers a paid app that is actually a pretty good deal. While it is on sale for US$0.99, Sleep Sounds HQ offers more than 600 relaxing sounds. Some of the categories are a bit weird, like Industrial and Trains; not the kinds of soundscapes I would think of first if I needed to get some rest. Still, there is so much to choose from and the lack of ads makes Sleep Sounds HQ seem the better deal.
I would suggest trying the free app and see if you like the sounds and features; if you do, migrate to the paid version. I am sure that was the developer's intention in the first place...
My favorite sleep/relaxation app is still Naturespace. It's a free app with a lot of in-app purchases, but the audio is first rate, and the free version has some great soundscapes. Naturespace is designed for headphone listening and has special settings for earbud listeners, but it sounds fine over external speakers.
Sleep Sounds and Naturespace both provide natural sounds, which I prefer to synthesized soundscapes. Free Sleep Sounds requires iOS 6 or later. It's not universal, and it is optimized for the iPhone 5.