Sound Timer is a free program that allows you to play a sound file at a certain time.
Type the location of the sound file in the top box, and the time to play the sound in the bottom box. Click on the tick to close the window - Sound Timer will stay active as an icon on the task bar.
To close Sound Timer completely, click on the cross on the titlebar.
You can run several copies of Sound Timer if you wish to play a number of sounds at different times.