Sorry you have had such bad luck.
I agree that that is the yolk which has not been fully absorbed and hatched too early.
A stethoscope may enable you to hear the chick trying to break out....tapping, but you won't hear the heart. If the chick has broken through the internal membrane it may cheep. The egg will probably move if it is near to hatching. However, if there is no sign of movement on candling it may not be alive....sorry.