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.