Apple did not include an obvious feature for several years – If you use your iPhone as your alarm, you likely have grown tired of starting your day to the same marimba tone. This was till iOS6 came out – iOS 6 allows you to set the alarm to play a song instead of a tone.
To set a new song as an alarm rather than the same old boring alarm tones, click the Clock icon and then go to Alarm. Add a new alarm or edit an existing alarm. Then go to Sound and click “Pick a Song.”
If you use iTunes Match and have it set to display both locally stored songs and those in iCloud (Settings > Music > Show All Music), you can choose a song from iCloud as your alarm. Be careful, however, with choosing a song from the cloud — if you lose Internet connectivity while sleeping, your alarm will be silent. A best practice here would be to download a song to your phone before using it as your alarm.
This feature is also available in the iPad with iOS 6 version.