Melting point of objects and food
As Poutnik said, the melting point is well defined for pure substances. In the case of egg yolk, it is not a pure substance and also when you heat it, some molecules decompose. So we can't measure or even define a melting point for it. Also, the temperature is not constant, when the egg is melting.
A substances' phase doesn't get influenced only by temperature. Maybe it is strange that heating the egg yolk in a pan, makes it to behave like a solid. Actually, some complex changes in intermolecular forces are getting done.
If you want to cause an egg yolk to melt, I think that the only work to do is to heat it under the normal pressure.