Austin and Memphis are about 560 miles apart by road. Split the difference and you arrive near Dallas, the city closest to the halfway point between them. That puts roughly 3 hr 49 min of driving on the Austin side and 8 hr 48 min on the Memphis side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Dallas suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Dallas doesn't have what you're after, Houston and Oklahoma City are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Austin and Memphis.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.