How tightly is your picade board screwed down?
I found it was very easy to screw it down so tightly that it was being slightly bent, and I had serious hissy sound in one speaker, and sometimes nothing at all from that speaker, until I loosened the screws a little.
I think the screw holes might be a fraction out and bend the board when fully tightened?
After loosening the screws, there was still a slight hiss a high volume, but not as bad as it was, and both speakers worked reliably.
Now I have switched to USB soundcard, with the output going to the picade board and then to the speakers, and the hiss is totally gone.