What's the best solution to boost my built-in speakers' max sound output level in Mac OS X?

"How About Rogue Amoeba's SoundSource? I'm not sure if it does exactly what you want, but give it a try. The price is right. Otherwise Audio Hijack Pro was going to be my recommendation.

For a hardware solution check out HeadRoom's Total Airhead or Total Bithead, which besides amping up the volume, it will help the sound quality if you don't have the most efficient headphones. This is very dependent on what headphones you have."

"I ran across this thread over at the Apple Support Discussion forums -- definitely looks like it's got potential. Will try it out and update this answer with the results. If anyone else tries this, please comment on your results.

Update:
I haven't been able to follow the setup instructions described in the thread -- I'm not very familiar with Audio Lab or MIDI configuration in OS X. Anyone else tried this?"

"Soundbooster sounds like what you're asking for. It's an application but it sits in the menubar, by default it does not show in the Dock, although it can do that too.

It has poor reviews on the App store but I'd say that it's because the developers were not clear about the install requirement on the tin.

The MacApp Store App requires drivers be installed to operate as intended. There is a lite version too if you don't care about the equalizer function or being able to control individual app volumes.

My personal experience is thus: I tested out the lite version, rebooted, saw that it worked, uninstalled using the script provided by the dev. Installed the Mac App Store version and installed the driver and rebooted. It works. The drawback I've noticed with this solution is that the Mac system volume control becomes non functional except for toggling mute on and off. In order to adjust the volume, you have to use the SoundBooster volume slider from the menubar. Unfortunately, the App doesn't have any hotkeys or shortcuts or command line access to make changing the volume more accessible."