Well,

I tried all 6 PCI slots and the result is the same with the 3 top slots. If I use the 3 bottom slots, the sound starts to be working ok but the computer freezes after a few seconds. It could be something wrong with my motherboard, or, as Unknown_K suggested, it is probably the issue with PCI in the 8x00 and 9x00 powermacs.

It's a bit frustrating because I feel that my computer has enough power to do great things like video editing (even with 640x480 files) but my SCSI drive is not fast/big enough and the SSD has this bug with audio. It's too bad.

It is still a pretty good machine for graphic design though (which was it's first purpose)!

Thanks for your help anyway!