Why have i lost start/shut down sounds

Do you see a speaker icon in the lower right of your desktop screen?
If yes, double click the speaker icon, check the volume and mute settings.
If no, click my computer, click control panel, click sounds and audio devices, check the device volume, check the box that says: "place volume icon in task bar", un-check the "mute" box, click the sounds tab, look at: "sound scheme" box.
Select the windows sound scheme from the box, click apply, click OK, close all windows, restart the computer.

Now suppose the sounds and audio devices in the control panel does not work, or is missing, the speaker icon is missing from the desk top, you will need to reload your audio drivers. Your audio drivers have been erased.

Look up your model number on the computer manufacturer's website. Download the audio drivers, turn off your anti virus, install the audio drivers, restart the computer, turn the anti virus back on. That usually fixes it.