How can I record a mp3 file on my laptop and use it as an alarm clock?
You can record it using a microphone or the Line in and some software which you will be able to find at http://www.download.com you can set it to play the MP3 at a specific time using your “Scheduled Tasks” which is located in the control panel, for this however you would have to leave your laptop running.
If you have the ability in BIOS you can set your laptop to turn on 5 mins or so before the alarm (depending on how long it takes to fire up) then your scheduled task will kick in when the laptop has loaded(use the Run at startup tab when creating the scheduled task)
Good luck.