If you're using the built-in microphone on an iOS device and are also using the built-in speakers for that same device, the audio captured by the microphone will play through the speakers and feed back to the microphone.
There are two ways to work around this issue:
1. lower the volume on, or completely turn off the sound coming through the speakers on the iOS device using the normal hardware volume buttons on the iPhone, iPad or iPod Touch
2. use headphones on the iOS device so that the audio does not play through the built-in speakers and cannot reach the microphone
NOTE: do not turn down / mute the built-in microphone from within the Live:Air or Live:Air Solo app, as that will reduce / prevent all audio capture from the built-in microphone. Instead, use one of the above methods.