How to pause windows shut-down

How to pause windows shut-down

I need to Mute/Un-Mute the sound card at start-up and shut-down.
I have found some code to do the work, but often windows slams through the
shutdown and the sound never gets muted.
Can someone please tell me how to pause the shut-down long enough for my
app to mute the sound. I can use a simple TTimer to pause the app long
enough for it to run the Muting and then let windows get on with shutting
down.
How do I tell windows to wait though?
I notice that if I leave Firefox running and try to shut-down, windows
stops with a message, "These programs are preventing windows closing..."
and needs a click to force Firefox to close. I need to find that.
Thanks