Jests, although at times they may be quite proper, should be altogether banished among clerics. The voice should be plain and frank.
102. Men of the world give many further rules about the way to speak, which I think we may pass over; as, for instance, the way jesting should be conducted. For though at times jests may be proper and pleasant, yet they are unsuited to the clerical life. For how can we adopt those things which we do not find in the holy Scriptures?
103. We must also take care that in relating stories we do not alter the earnest purpose of the harder rule we have set before us. “Woe unto you that laugh, for you shall weep,” says the Lord. Do we seek for something to laugh at, that laughing here we may weep hereafter? I think we ought to avoid not only broad jokes, but all kinds of jests, unless perchance it is not unfitting at the time for our conversation to be agreeable and pleasant.
104. In speaking of the voice, I certainly think it ought to be plain and clear. That it should be musical is a gift of nature, and is not to be won by exertion. Let it be distinct in its pronunciation and full of a manly vigour, but let it be free from a rough and rustic twang. See, too, that it does not assume a theatrical accent, but rather keeps true to the inner meaning of the words it utters.
Source: On the Duties of the Clergy (New Advent)