I found 3 options on my macbook pro. I’ve got Snow Leopard
1. just like b said, change the equalizer. in iTunes on Snow Leopard it’s Window -> show Equalizer or (control + Apple + 2). This option adds 12 decibels.
2. In iTunes, go to “preferences”, then “playback”, then un-check “Sound Check”. This gave me a few more decibels louder. Obviously if you are listening to different songs in iTunes and they are recorded at different levels, then there could be significant volume differences when you un-check this box, but it can increase your volume.
3. Once again, in iTunes, highlight a song, press apple+i to show the song’s information, than go to “options” and there is a volume adjustment you can make from -100% to +100%. This option would be annoying and probably not a great idea to do to all your songs, but if you’re only working with few songs it could be helpful.
(by the way, I’m not responsible for anybody who blows their speakers using these ideas, I’m currently trying them so I can hear my computer in my kitchen, we’ll see how it goes!)