Kris Kasprzak built a microphone monitor to set audio levels during video recording on a DSLR camera.
This DIY project displays the spectrum or a microphone input for 32 bands. Includes a line in for powered microphones, a line out for connection to a camera, and an audio jack to monitor the audio with headphones. Through the touch screen display you can adjust the audio levels as well as display settings. All the settings are saved so that when you turn the unit back on, the settings will come back just as you last had them.
Code for the project can be found here.