Sorry Dicho, but I have to disagree. The English translation of Alabaster's Roxana was made at around the same time as the Latin drama, so that Mrzzy's instinct that the phrase dated back to the time of Shakespeare and the Authorised Version is vindicated.

What is also quite interesting is that the Latin is almost a direct quotation from the Ars Poetica of Horace l.323
Graiis dedit ore rotundo Musa Loqui
(Trans)The Muse of Speech gave elegant voices to the Greeks
I would guess that there was a 16th century translation of Horace which gave ore rotundo =elegant, or polished voice as "Pear-shaped tones.