For starters, not everyone can act or has the right attributes to deliver a good enough performance or at least has a voice good enough. Just record your voice and play it back. Does it sound right? Too high/low?
And second, recording is not as easy as it seems, should TT pay the trip to the studio? What if the winner is not from the US?
Sending a mp3/wav with your line(s) is not an option unless TT spends budget/time editing/de-hissing/filtering/etc the lines for a cameo character.