I don't know so much about wheelbuilding, being rather a novice, but was taught that the purpose of the tension gauge was to make sure that the spokes are tensioned enough (that is, each spoke has more than a minimum amount of tension), and to attempt to get approximately the same tension on each spoke around the wheel. Yes, you can do this by sound ... but it's much harder to teach/learn "ping" not "ding" than it is to say "measure each spoke, take the average, if the average is >=20 and no spoke is out of the range 20+-2, and it's round, dished, and true, then you're done."