Hello,
http://lipsofsuna.org/wiki/audio-tasks lists heartbeat as low health warning, which - without having tested it to be honest - I'm not a fan of. I imagine it would be very annoying when you just re-spawned and have 1hp to hear the warning and when you got out of danger but still have low health.
A more subtle way to indicate low health could be to have the health bar pulsate.
Here is a mockup made with GIMP/Kdenlive:
http://www.youtube.com/watch?v=ghtZkK1RXv0
I don't know if that's a reasonable suggestion for how the user interface is implemented.