Guys,
The reason why the ambient sound level is reduced is that the R recirc fan in addition to the R pack supplies the cooling air for the overhead cb panel (that annoying icy blast when you are on the jumpseat) and forward instruments in addition to windscreen demisting.
On the ground with the APU supplying air the pack will already be in Hi flow and therefore the noise difference will be quite marked. In the air turning the R recirc fan off THEN puts the pack into Hi flow to compensate and the noise difference will not be so marked.
Hope that helps