I can't here anything other than 'ok' however that ok too seems to be in a different tone compared to the other transmissions, very rapid response.
I've had a listen to the original Liveatc audio and can't hear it there either, however this transmission was less than 60 seconds prior to the first "Giant 3591 Houston Approach?"
At 8.35 on the audio file is the ok message, the atc request is at 9.25, the first ELT request at 10.05. Would interesting to know if the first "Giant 3591 Houston Approach?" was because the control had lost the aircraft on radar or because he noticed it was in a dive/descending below assigned alt.
http://archive-server.liveatc.net/ki...2019-1830Z.mp3