Well, i would guess that the problem is with the Sproggy power supply. It sounds like its not giving enough AMPS on one of the supply lines. I would think its 12v that isnt high enough, maybe 5v, but i dont really know at all which one it would be.
What type of speakers are you running from it? Do you have the speakers directly hooked to the sound card? or do you have a separate amp?
If you have car speakers, you need to have an amp to power them. The amp that is onboard of the sound card is only a preamp, most likely not even 1 watt. To power car speakers you probably need at least 40watts.