Who gave David the rehearsal message

Not spoilers. You assume it's a broadcast... INSTANTANEOUSLY reaching them with no time delay. (notice that in Aliens, it takes 2 weeks to get an answer to broadcasts... approx on week for signals to travel about 39 light years.... still WAY faster than light... but a delay none the less.) It's probably an interactive hologram with voice recognition technology. It plays a segment and if David replies correctly, then it plays the 'perfect' message. Nothing indicates that the teacher who recorded it would have to be told what it was for, just 'here's some money, record a language course on that ancient language from your part of the word '- seemed to be India. Presumably about a dozen other dudes doing the same for the other ancient languages.

this program ist just a language training as David has to keep his brain cells fit on board, 2 years alone, as the ship butler..