The Mixer Toolbar has three controls, used to set the volume levels of
your audio device and choose the input source. The leftmost slider
controls the output volume, the other slider controls the recording
volume, and the control on the right lets you choose the input
source (such as "Microphone", "Line In", "Audio CD", etc.). Use the
Record Level Meter to set the correct
level.
Changing these controls has no effect on the audio data in your project - in other words it doesn't matter what the output volume level is when you Export or Save a project - the end result is the same.