Yes, you read the title right. I was surprised myself to run across his comments in City of God 14.24, but it’s there. A little background first. In book 14, Augustine is observing the fact that for the most part, our will controls the movements of our bodies. However, the one startling exception is that of sexual arousal. This, he contends, is not controlled by the will but by lust. This, of course, fits into his whole scheme of original sin, traducianism (=soul passed on from parents), and concupiscence (=disordered desire). It should be noted that Augustine did not view sexual intercourse as the cause of the fall, but the fall corrupted sexual intercourse. In this case, the in ability for the appropriate bodily parts to be in submission to the will.
In chapter 24, Augustine is highlighting all of the amazing feats that people can do by the power of their will. One person can swallow something and voluntarily bring it backup. Another can imitate the sounds of animals. Or some can even sweat on command. So on and so forth. Then comes the line on farting:
A number of people produce at will such musical sounds from their behind (without any stink) that they seem to be singing from that region.
Augustine, City of God, tr. Henry Bettenson (New York: Penguin, 2003), 588.