I tried playing an MP3 file with VLC player and iTunes both with largest volume of the player and the Mac OS X system. The former seems to play a little louder (any way to quantitatively measure?) I am wondering if there is anything that only involve software settings I can do to make it sound louder.
In VLC select
Tools->Effects and Filters->Audio Effects->Graphic Equalizer->Enable then increase the preamp and/or band sliders. Also try using the 2-pass option (it seems to work better for multi-channel DVDs than for two-channel video files/MP3s).