Canon Rebel T2i Quick Start Manual page 81

Audio Output Level Adjustment (Pad) and Battery
The microphone requires a 9 volt
battery (not included).
In this compartment, with a magnifying
glass and a pin, the output level for the
microphone can be adjusted.
Arrange the two switches to correspond
to the desired output level.
The camera has no input level meter, so
how do you know which microphone
output level is correct? Read on.