MenuBarAudio is a system utility for users with multiple audio devices. Users of Mac OS X 10.5 and 10.6 who have multiple audio input and output devices usually face a problem when they want to switch between those devices. They have to go to the preferences and click many times to find the correct place to change the device. Users of Mac OS X 10.7 (a.k.a Lion) can simply change the input device by holding the "alt" key and clicking on the speaker on the Menubar. MenuBarAudio replicates that functionality for those users who don't have Lion installed. It puts a new icon on the menubar. You can simply click on that icon to switch your audio input or output device. From the menu, you can also access the Preferences window, where you can change the default device. There is also an option that lets you restore the input and output device after you quit the application, which is ideal if you want to use the app occasionally.
On the Mac App Store description, it is stated that even after you quit the application, you can alt+click on the speaker icon and you will be able to change devices as well.