The only good preacher is a dead preacher

"The preacher is like an ass.  So long as he lives, he produces a repulsive bray.  [But once] he is dead one can make out of what is left of him musical instruments that produce a beautiful sound.  Conclusion:  the preacher must be a mortified man!"

     Peter Damian, De sancta simplicitate scientiae inflanti anteponenda 4, [Quid in concionatore requiratur [in a preacher]], as translated into German by Otto Hermann Pesch, "Theologie des Wortes bei Thomas von Aquin," Zeitschrift für Theologie und Kirche 66, no. 4 (1969):  447 (437-465).  De divina omnipotentia e altri opuscoli, ed. Paolo Brezzi, trans. Bruno Nardi, Edizione nazionale dei classici del pensiero italiano 5 (Firenze:  Vallecchi Editore, 1943), 174 (163-201):

Tu quoque, fili mi, ut praedicandi sortiaris officium, imitare vel asini, de quo dictum est, vel etiam ovis exemplum.  Quae nimirum cum vivit, turpiter beat, mortua vero in instrumentis musicis suaviter cantat; sic et qui carnaliter vivunt, balare cum ovibus inaniter possunt, naviter explere praedicatoris officium nequeunt.

You, too, my son, in order that you may be allotted/obtain the office of preaching, imitate the example of the ass, as they say [(de quo dictum est)], or even of the sheep.  Which, of course, while it lives, gapes [(beat < beo = Fr. avoir bouche ouverte, béer > bayer acc. to Blaise)] in an unsightly manner, [but once] dead, in [the form of] musical instruments sings very beautifully.  So also those who live carnally are able to bleat with sheep in vain [(inaniter)], [but] are wholly [(naviter)] unable to perform the office of preacher. 

Also  PL 145, col. 697.
     See also here.
     Although Peter speaks of instrumenta musica, he seems to have mainly those that can be made out of a corium (skin or hide), for example the tympanum (tambourine or kettle drum) and the stringed instruments supplied with chorda (cat gut).
     In sec. 3 he says that "God omnipotent does not need our philology [(grammatica)]", as evidenced by the fact that Samson converted (figuratively speaking, of course!) a thousand Philistines by flinging about (iacere can also mean to let fall in speaking or utter, though Peter does not draw attention to this) the jawbone of an ass.