Usually we use sound to describe an argument or proposal; also to refer to a person's health and mind. When people make out their will and testament for their eventual death, the document will say "I, <name>, being of sound mind...." meaning being of a healthy reasoning mind. I can also say "Your argument is sound, you have convinced me." A 'good' pronunciation, perhaps 'excellent' or 'very good', will do just fine.

While agreeing with fdmaxey , i'd like to add that "his pronunciation is sound" is also right. This sentence refers to a person's ability in general to pronounce words in a particular language .

Dictionary definition of 'sound' Noun: Vibrations that travel through the air or another medium and can be heard when they reach a person's or animal's ear. A long surgical probe, typically with a curved, blunt end. A narrow stretch of water forming an inlet or connecting two wider areas of water such as two seas or a sea and a lake. Verb: Emit sound. Ascertain (the depth of water), typically by means of a line or pole or using sound echoes. Adjective: In good condition; not damaged, injured, or diseased: "he was not of sound mind". Adverb: Soundly. Synonyms: noun. noise - voice - tone - strait verb. ring - resound - probe adjective. healthy - solid - strong adverb. fast - tightly - firmly - soundly - deeply