Full Idea
A theory is 'sound' iff every theorem of it is true (i.e. true on the interpretation built into its language). Soundness is normally a matter of having true axioms and a truth-preserving proof system.
Gist of Idea
A theory is 'sound' iff every theorem is true (usually from true axioms and truth-preservation)
Source
Peter Smith (Intro to Gödel's Theorems [2007], 03.4)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.24