Firstly, create a new event in iCal, at the time you want the file to play. In the info drawer/window, set the alarm to "Open file". From the menu that appears below it choose "Other" and then find the music file. Then set the alarm to run 0 minutes before the event.
If the music file is set to open in QuickTime Player, open QuickTime's Player Preferences and make sure "Automatically play movies when opened" is checked. If it's set to open in iTunes it should automatically play when opened. Otherwise, check the settings of whatever application it's set to open in or set it to open in QuickTime Player or iTunes.