OpenSL ES is a relatively new addition to the Android ecosystem. It was first added with the release of Android 2.3.3 (version 1.0.1) and has seen a relatively slow uptake since that time. What it offers is a similar feature set as the more well-known and wider supported OpenAL: full 3D audio capabilities with the ability to mix and otherwise manipulate audio using a system of sources and sinks.
An interesting feature of OpenSL ES is that it also includes microphone input support, allowing you to add such a feature to a game without having to integrate yet another API.
A disadvantage of OpenSL ES is that it's fairly complex to get started with, involving a fairly significant learning curve. On the other hand, it is a native API on Android, so it will be available if the the Android version is 2.3.3 or higher. This is a distinct benefit over OpenAL, which is neither supported by nor easy to integrate with, Android.
There's also the issue that OpenSL ES has no Java API on Android. In...