Augustine and the Calvinistic tradition in general define the will's freedom, or lack thereof, in relation to sin. Why? Because this is how the Bible defines it. Jesus declared "everyone who practices sin is a slave to sin. ... So if the Son sets you free, you will be free indeed." (John 8:34-36) Augustine understood that before the fall, Adam was "able to sin and able not to sin", that he, as representative of the human race, was in a probationary state, not sealed in righteousness (like the glorified saints). Likewise regarding man's condition after the fall he said we are in the sad condition of being "not able not to sin" So Augustine understood the Bible to be teaching that Adam (pre-fall) was free in regards to sin's bondage but his willful act rendered his post-fall descendants to be in bondage to corruption; to have a will that is no longer free at all (apart from grace) to make God-pleasing redemptive choices. It is worthwhile to remember this in your discussions about free will, because the historical debate about free will refers to man's condition in sin after the fall.
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Related Resources
Human Nature in Its Fourfold State Thoughts on Augustine's View on the Will
Human Nature in its Fourfold State (eBook) by Thomas Boston
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Quotes
"To will is of nature, but to will aright is of grace." - Augustine
If man has lost his freedom, and is forced to serve sin, and cannot will good, what conclusion can more justly be drawn concerning him, than that he sins and wills evil necessarily?" - Martin Luther BW pg. 149