Ok, here's what you do. For instance, let's say you wake up at 7:15am.
1. System Preferences > Energy Saver > Schedule...
2. Set the schedule to Wake at 7:10am (to allow time for computer to awake from sleep mode)
3. Open iCal and create a new calendar set; File > New Calendar > Name it "Alarm" or "Wake Up" or whatever
4. Create a new event:
- Double-click on day
- Input 7:15am as the time
- Select a repeat of every day
- Assign it to the Alarm calendar you created
- Alarm: Select Open file > Find the music file you want to play as the alarm, and click OK
- Put 0 for the minutes before box
What you've done:
1. Instructed your computer to awake from sleep at 7:10am.
2. Created a new calendar to organize your wake-up calls.
3. Created a new daily event that will launch your selected song at 7:15am (regardless if iTunes is open or not) and play.
Now, in order for this to work properly, you must make sure your speakers are on and that you set your computer to sleep and not shutdown. Alternatively you could just leave it powered on as I do.
Additionally, if you insist on wanting the computer to shut down, make sure you give it enough time to boot, so I'd set the start time in the Energy Preferences panel to 7:00am. Also, this isn't going to work if you have a password set; otherwise it can't continue, so you'll have to disable your password. I'd recommend just putting it to sleep or leaving it on to avoid such hassles.
Hope this helps.