Pepanee & Cereberus: Sorry I ignored you both and replied to Fireberd. I somehow didn't notice that there was a second page to the thread
I saw that Techspot page when I searched before. It lets you download the installer for the installer but when you run the installer it requires a serial key, which I don't have.
Free Video Volume Booster's page says it works for "AVI, DIVX, XVID, MPEG, MPG, MKV, RM, RMVB, FLV, MOV, MP4, ASF, WEBM". No mention of MP3. But there was a link on the page for Free MP3 Quality Optomizer and that says it will "boost the volume". If I am going to install something to do this I would rather it be capable of doing other things too (like clipping out parts I don't want e.g. the occasional long silence) but I have saved a shortcut to it in case I can't find something more capable.
Fireberd: There are lots of free (or better yet contribution supported) applications that work as well as or better than many of the programs that demand that you pay a fixed price. I don't mind donating for something I use a lot but for something I only use occasionally I would donate less.