I have not had any problems with the alarms not going off, but i also have my phone set to not go into sleep mode if plugged in. I noticed that MP3's cant be used for the alarm. What i did to get around this was start playing a custom MP3 ringtone. and then use the record function on the phone to record it as it plays, as long as the volume is not too high while you're playing it the WAV created will sound fine. just take that wav and drop it in the windows folder and it will be on the list for alarm tones.
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Almma live