Do you know a simple sound booster app for Mac?

Boom

It works pretty well. I had similar issues on my Mac, didn't like being glued with the headphones forever so I got this just yesterday. And I loved it. It's not free, but for $5 it works like a horse.

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You can boost the volume of videos playing in Chrome using free extensions. I found the Ears chrome plugin works quite well. Steps:

install the plugin
click the Ears extension icon in chrome
drag the baseline from 0 up to +5
click EQ this tab

You can go with custom equalizer and then amplify your sound out

Instructions

Requirements

Soundflower – free download from Google Code (more recent version here)
AU Lab – free download from Apple Developers (requires free Apple Dev ID)
Download and install both Soundflower and AU Lab, you will then need to restart your Mac to have full access to the audio components. Once rebooted, follow along with the instructions below:

Set Up a Universal Audio Equalizer for Mac OS X

Set System Volume to the maximum level, do this either through the menu bar or by hitting the Volume Up key repeatedly
Open System Preferences from the  Apple menu and select the “Sound” panel, followed by the “Output” tab. Select “Soundflower (2ch) from the Output list
Now launch AU Lab, found in /Applications/Utilities/
From the “Audio Input Device” pulldown menu, select “Soundflower (2ch)”, and then from “Audio Output Device” menu select “Stereo In/Stereo Out”
Click the “Create Document” button at the bottom of the screen At the next screen, look for “Output 1” column and click the “Effects” dropdown, selecting “AUGraphicEQ”
This is your new system-wide equalizer, set it how you see fit. Changes here will impact all audio output on the Mac
When satisfied with the EQ settings, hit Command+S to save the EQ settings file and put it somewhere easy to find like the Documents folder Now open AU Lab preferences from the AU Lab menu, click on the “Document” tab and click the radiobox next to “Open a specific document”, selecting the .trak EQ file you saved in the previous step
Optional final step: If you want the EQ settings to load on every Mac OS X boot, right-click on the AU Lab icon, go to Options, and select “Open at Login”

Note: It’s important to note that AU Lab must be running in order for the equalizer to have an effect, keeping it running will consume a small amount of CPU resources but it’s much less process hungry than some of the third party alternatives available on the market.

Due Credit : http://osxdaily.com/2012/05/18/equalizer-for-all-audio-mac-os-x/

Cheers Equalizer