take a computer microphone like a cheap one any mic you can plug into a computer.
press it to the middle of your chest.
use computer o record.
listen to afterward.
that's all i could think of to say.
Have a bath, and stick your head under water (only to cover to your ears obviously, so you can breathe). Relax, and you'll be able to hear your heart beat. It really works, so try it.