Listen to audio books and music through your phone's earpiece when you don't have earphones handy. Boost your audio (Android 2.3 through 4.1 or so only, not working on 4.2.1 or higher), whether through earpiece or through the speaker.
Earpiece is a simple, light-weight audio-control app.
You can pop Earpiece up on top of another application simply by holding down the SEARCH key.
In the options, you can enable an experimental feature which switches the phone to speaker phone when it's away from the face and then back to speaker phone close up. This only works on devices with a proximity sensor, of course.
Another experimental feature is that you can significantly reduce the volume of the camera sounds on some devices. Ensure your local laws allow you to do this.
Earpiece is free and Open Source.
USE AT YOUR OWN RISK. Playing audio at high volumes, especially for a prolonged amount of time, can damage speakers and/or hearing.
Note 1: There is no affiliation between the developer of Earpiece and the developer of Mortplayer, which is the application shown in the background of the screenshots.
Note 2: The reason the application needs permission to read log data is for the camera quieting feature. When the camera quieting feature is active, Earpiece monitors the log for when the camera is about to take a picture and then quiets things for about two seconds. If you are worried about privacy issues, or otherwise curious, you can look at the full Earpiece source code at github.com/arpruss/earpiece and you can build it from source yourself.
Note 3: If you use Earpiece, don't use my Speaker Boost with it--just use Earpiece's audio boost option instead.