Maybe the alarm volume level is low. All you have to do is to go to the Settings and move the volume slider to the right.
Go to Settings and tap on Sounds (& notifications). Move the slider under Alarm volume to the right as much as you prefer.
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Note: The screenshots are taken using a Sony Xperia M2, running on Android 5.1.1 Lollipop. The instructions mentioned here might slightly vary from one manufacturer to another.
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