Speakers make sound rigth after pc shut down

Hello

Hopefully some of our members will chime in soon..

@PeterOz

I had a similar noise when shutting down one of my PC's years ago.
it was just capacitors discharging after power was cut and causing electronic interference that the speaker circuity was picking up until it too completely lost power.

changing those two bits of hardware would be the cause, either they are pumping out more interference or unshielded cables are closer together.

while I'm no expert in the field, I would be really surprised if it was in any way harmful.
and I base that theory on building PC's since 1988 and being a hobby enthusiast with electronics since 1979.
as I said, mine did it for ages until I upgraded things and the noise went away.
I'm as certain as I can be that you are just witnessing the side effect of the internal electronics discharging and the sound system picking up that discharge.
you have upped your PSU wattage so more current is flowing around, add to that you would have unshielded wires close to the new hardware and things can be explained away very quickly.
it only happening when you shut the PC down also supports my theory - any electrical circuit suffers their biggest stress when turning on and shutting down. it's one of the original theories why computers were left on 24/7, as mainframes still are.

try move any wire associated with the speakers as far as way as possible from anything else.
obvisously some things won't be able to be moved, but see what can, and if it helps.

while I would like to say it should, and probably will, sadly only time will tell.
good luck with the rest of the upgrades.