Perhaps they had a clear tape-recording of same.
I'll bet it kept everyone awake.
I once played the US national anthem over the PA in a 1011 after a long red-eye ferry flight (where all the cabin crew could sleep, but we had to stay awake). It was recorded on my little mini-cassette recorder, and came over fine (except they were peeved).
I only wish I had 'Rule Brittania' recorded, as that would be a lovely tune to wake up to!