If the processor was not inserted correctly the machine would not do anything. Video going might be a VRAM problem (there are 4 1MB VRAM slots on an 8500, installed in pairs). If sound is going then you probably have a problem somewhere on the motherboard (leaking capacitors, something shorting out) unless you took the speaker cable off the motherboard while tinkering and forgot to put it back on.

Have you tried a mac PCI video card to see if it works?