Sound heart - literally, “heart of health,” that in which all emotions and appetites are in a healthy equilibrium. The contrast with this is…

A sound heart — that is, one in healthy condition, of which the passions and emotions are under control.

A sound heart [is] the life of the flesh
A heart made so by the grace of God, in which are sound principles of truth…

An upright, contented, and benevolent mind tends to health.