Might sound stupid, but have you used the correct soundcard socket for your speakers' feed? Some cards have several jackplug sockets.
It could also be that your bass speaker (i.e. sub woofer) requires a separate power supply.
Other possibilities include inadvertently muting one or more of the line outputs in the volume mixer, not turning the mixer's sliders to maximum output or using the mute facility in, say, Windows Media Player.