That's what the cockpit voice recorder (CVR) is for. It records, on a loop tape, every sound in the cockpit (including, of course, the pilots' voices), and has already been recovered by the NTSB. The ill-fated AA MD-80's CVR, in fact, is currently being analyzed by the NTSB in Washington - it was recovered yesterday. Additionally, there's a room in every ATC facility that stores huge tape recordings of radio transmissions, and the NTSB, I'm sure, will be using the recordings of both Little Rock Approach and Little Rock Tower to hear ATC exchanges between AA1420 and ATC. Hope this helps.