Aw geez that's what I get for posting before coffee time; I totally misinterpreted your question, my bad.
I'm curious what is playing too loud. If it's iTunes, web browser video, Garageband, etc, (ie anything with a graphic interface), you could just use the volume slider specific to the interface you are using, and turn down the volume that way.
If that doesn't do it, then try holding the shift and option keys while lowering the system volume further. You will be able to lower it to 1/4 of 1 standard volume bar, giving you more control over the lower range.
If you really think that is still too loud, you might have to just get some external speakers, and use those to lower the volume output even further.
2009 13" MBP 2.53GHz CPU, 250GB HD, 4GB RAM
iPhone 4 16GB