It generally happens on the taxi out to the runway, and therefore coincides with the Crew/Auto safety demonstration. If it's the latter, this can often be completely inaudible. On one of my recent flights as nobody could possibly have heard it, and I almost got up and said that it should be played again, but by then of course it was "Cabin Crew seats for take-off". What would you have done in a similar situation, as it rendered the mandatory safety demo completely useless?