Sound Booster
Note: free Lite version is available, see http://froyosoft.com/blog_post.php?post=50 for more information.
Sound Booster is a simple yet powerful app for an enhanced audio experience on your Mac. Through boosting the volume of music files, you can enjoy your music with a boost on your Mac, iPhone, or iPad. Additionally, with an optional component, you are able to boost the volume of all apps on your Mac, enjoy a system-wide equalizer, or fine tune the volume of individual apps to your liking. Once you try Sound Booster, you'll wonder how you ever went without it! Combining the best features of several audio modification apps on the Mac, Sound Booster is the only app you need to get everything you want out of your Mac audio experience.
