There are two relevant *software* volume controls to consider: Master and PCM Media-players usually manipulate PCM without touching Master. First make sure your Master volume is not too low using aumix(ncurses) or xaumix.
Secondly it is possible that your speakers need power-supply for amplification which you have not connected. When I use my speakers without the A/C adapter and the speaker switch set to "OFF" position, I get just enough volume for one person to hear. The sound card o/p typically provides just enough power to drive ear-phones and not a ~100W speaker (which reminds me ... do you face same problem with earphones??).
- farazs