Get this extension! It is excellent! Here is why:
- works perfectly
- open source
- streamlined
- contains extensive documentation
- minimal permissions required to function
- the developer is very active (see other review for his/her responses and fixes)

There are many extensions that provide volume boosting functionality. The ones I have tried are completely untrustworthy. The main issues I have are things like: ad injection, NOT open source, pure copycats of other extensions, and so forth.