The 4 beeps/flashes indicate a fault that is getting back to the power supply, and it can be from any connection. It is a PITA to figure out, but you can debug this by disconnecting everything (down to that faulty speaker, where I'd start first), and I mean everything, and then slowly start adding things back in. You want the power supply disconnected from everything and everything disconncted from the motherboard.
Any connection that the power supply perceives as a short or causing a too-low resistance event will trigger this.
So, set aside an afternoon and get out your flashlight and take some notes..... I have done this and generally have found it is not the motherboard at fault.......