When in the timeline you can hit Option - W to bring up the Clip Overlays which show you the red lines that indicate audio levels or opacity on video clips.
You can drag these up and down to change the audio level of a clip but you can also do it to several clips at once if you use a key command.
01) Highlight the clips you want to adjust the audio levels of.
02) To increase or decrease them one unit at a time use Control + + (plus sign) to increase or Control + - (minus sign) to decrease.
If you want to bump them up by 3 units at a time instead of + and - use [ (left bracket) and ] (right bracket).
You can also - while you have clips hightlighted- hit Command + Option + L to bring up the gain adjust window to adjust them all a certain amount or a relative amount.