Useful tips to never miss an alarm:
Keep your computer plugged in and never close the lid.
Make sure your external speakers are on, otherwise unplug them.
Set OS X scheduled start up time 5 minutes before your alarm, if you are planning to shut your Mac down or switch to a sleep mode (System Preferences -> Energy Saver -> Schedule...) and make sure that "Turn Display off after" is set at least to 10 minutes.
Why it is important (My Alarm Clock can't have control over OS X & Hardware limitations):
1) Alarm won't go off if you put your Mac to sleep mode or shut it down.
If you'd still like to use the sleep mode there's another way to ensure that My Alarm Clock will sound on the required time. Set scheduled start up for your Mac:
1) Open System Preferences on your Mac
2) Go to Energy Saver
3) Make sure that "Turn Display off after" is set at least to 10 minutes.
4) Click Schedule
5) Select "Start up or Wake" and set it 5 minutes before the time you need to be woken
Your Mac will start up automatically letting My Alarm Clock wake you up.
2) Never close the lid of your Macbook. With the lid closed your Mac enters the "deep sleep" mode and is not able to wake itself up, even with the scheduled start up.
3) OS X will send the sound to the peripheral device currently in use. So if you are using external speakers, make sure you have turned them on, otherwise unplug them.
How to:
Set Alarms
To quickly create an alarm click the bell icon in the top right corner and then the Add Alarm button. You'll see the section where you can add alarms, set their frequency, snooze time, sounds, and adjust the volume level. Feel free to add a note and use the alarm as a reminder. The total number of active alarms is shown in the red circle next to the icon.
Set up Sleep Timer
Sleep Timer will help you fall asleep. Set a period of time and choose music you'd like to fall asleep to; the music will stop after the the set time interval expires. You can use existing sounds or choose music from your iTunes library.
Enable Nightstand Mode
Click the nightstand icon and the app will switch to the Full Screen mode turning your Mac into a nightstand.
Get into Settings
The gear icon lets you into the application settings. Skins, Weather, Sounds, and Advanced settings are available here.