1
 $ echo 'play alarmclock.wav 2>/dev/null' | at 07:30 tomorrow

Feb. 4, 2012, 11:03 a.m.Anon5MAQumYj

Explanation

at 07:30 tomorrow schedules a job for 7:30 AM the next day, running whatever command or script is fed to it as standard input. The format for specifying time and date is rather flexible. http://tinyurl.com/ibmdwat

echo 'play alarmclock.wav 2>/dev/null' | feeds the play alarmclock.wav command to at, while 2>/dev/null causes the text output of play to be thrown away (we are only interested in the alarm sound).