True, but if it's thin enough, it will be negligible. At least that's what I was told by Dean Zelinski in regards to how DBZ Guitars are finished.
Or you could oil rub them and have no real affect on the wood that's not cosmetic.
OK, so it seems we've finally assessed that applying some sort of finish on the inside might not have as great of an negative impact on the sound as we thought. However, one still essentially won't really see the insides of the material once the headphone has been assembled. On the other hand, if the inside is finished, the wood is more protected from humidity and dry swings, which could lead to expansion and contraction of the wood - and possibly even cracking.
My daughter is a violinist - and has been for the past 10 years or so. Even though our house is humidified in the winter (we have 2 zone heating - one gas, one heat pump), her violin can get radically out of tune in just half a day from the movement of the wood. This eventually leads to strings popping or even breaking. The winter months can be especially hard on musical instruments. However, for the price that was paid for her violin, I'd suspect that they would have finished the inside with some sort of sealer. But, my guess is still that in a scenario such as that, cost isn't too big of a factor when you consider the complete price of the violin. There still must be some reason why they didn't seal the inside.
Edited by wje - 6/6/11 at 8:50pm