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.