Have you tried to plug speakers into the phone while the alarm is going off? I don't see why that wouldn't work.
Also, you might want to see a doctor about your sleep problems. I have a sleeping disorder myself and I'm a heavy snorer, but I can still hear my phone alarm even when it's at the lowest volume. A friend of mine has sleep apnea and he stops breathing several times throughout the night. That doesn't really have anything to do with your issue necessarily, i'm just saying that sleeping disorders can be serious. I'd encourage you to talk to your doctor and possibly take a sleep study.