Can I increase the level from my USB microphone? I have this usb headset: http://www.plantronics.com/us/product/audio-478

With the level turned all the way up in System Preferences, it's not loud enough for some uses.

Is there a way to increase this level?

10.10.3

2014 15" Retina
You may use volume booster software like
Boom
Or
In iTunes Library select media with low volume and press cmd+i . Go to Options and make Volume Adjustments.
Or
VLC allow you to increase volume to 200% by simply scrolling all the way up.