Letasoft Sound Booster enables increasing volume on your computer to up to 500% of the original maximum.
The program works by integrating with Windows (installing a new Audio Processing Object, if you're interested), which means it should automatically increase the volume of all your applications, no other setup required.
Basic operations are very simple. Tap a system tray icon, a volume slider appears, and you increase it to whatever you need. This should work immediately everywhere else.
You can also increase or decrease volume via a configurable hotkey.
The default settings should work for most people, but if you have problems, the Preferences dialog allows choosing a different boost mode. We had no idea what might be best - should you use "injection", or not? who knows? - but it's easy enough to try and option and just see what happens.
Please note, the trial version is fully featured but has one minor restriction: every ten minutes it stops amplifying sound for around three seconds.
Letasoft Sound Booster noticeably increased system volume for us, but the higher we went, the more crackles and distortion we noticed. We managed to amplify volume by around 25-30% without any issues, and increases of up to 100% were acceptable for undemanding applications (watching TV). That's worth having, but you might be able to achieve something similar by installing a free audio equalizer.