You can open the mixer by clicking on the mixer trigger above the track list.
Shortcut [M].
In mixer, you can:
- Control the volume and pan
- Activate solo
- Arm record for audio tracks
- Edit track name
Pan, Volume, and meter
Pan
Drag horizontally to pan, double click to reset back to center.
Volume
Drag the grey thumb horizontally to change volume, double click to reset back to 0 db.
If you want to put a gain on a singer track, the better way is to use the compressor from the fx tab. For more details, please refer to 
VocalEffect.
If you want to put a gain on an audio track, the better way is to modify the gain of each audio clip. For more details, please refer to 
Audio viewer.
Master
Master track is located at the top of mixer.