My guess is that the OP was concerned with the expansion and contraction of the rackets in what he thought was an un-pressurized cargo compartment. And as pointed out as well the pressure in the main cabin is still less than at sea-level so the same problem would have existed.

If the stings were natural gut, then maybe the dryness of an airplane might have a slight effect on the strings.

This seems to be a lot about nothing since the cargo compartment is pressurized.