I'm going to resist the easy Spinal Tap reference and skip right to the part where I tell you Boom is a Mac app that can increase the volume of your Mac's speakers. The app costs $6.99, but its developer, Global Delight, offers a free, seven-day trial. If you are often underwhelmed by the audio output of your MacBook's speakers, Boom is worth taking for a spin.
The free trial is available at Global Delight's Web site. After installing, the app places an icon in the menu bar. Clicking on it reveals a simple volume slider. Boom doesn't replace your Mac's default volume control; instead, it's meant to be used alongside your (presumably maxed out) default volume control. In a brief test using tracks meant to be played loudly from Appetite for Destruction, Boom increased my MacBook Pro's volume significantly, and without any perceivable loss in clarity.
Click the button below Boom's vertical volume slider to open Boom's control panel. You'll see two tabs in the lower-left corner: Mac Volume and Boost File. On the Mac Volume tab, you can turn Boom on and off and adjust the volume. You can also adjust the equalizer, including choosing among eight presets.
On the Boost File tab, you can drag and drop files to boost their volume. I played with this feature for a bit and found that you can easily "overboost" a file, if you will. You can adjust the level of boosting with a slider before clicking the Boost button. Your original files aren't changed; Boom creates duplicates with a "_boosted" suffix or whichever naming convention you choose via Preferences. By default, boosted files are added to an iTunes playlist titled Boom.
The primary thrust of Boom, however, is not boosting files but boosting the volume of your MacBook, which it does with aplomb. The free trial is worth checking out. Also worth checking out is Skip Tunes, a highly convenient audio control for iTunes, Rdio, and Spotify users.