Often the solution that uses the least amount of material / mass has the least impact regardless of material.

Example “paper vs. plastic”. While plastic bags blowing around the environment are bad for wildlife, if properly disposed of in landfill they don’t have this impact. Plastic bags use little material in comparison to paperbags and depending on how far they are transported this energy use outweighs the benefits of recycling.

Further, its not clear that using wood is in fact a good idea since it can lead to deforestation.

The amount of wood and the volume of prodution of these headphones (100K or so) is likely not to have much impact say compared to the use of disposable wooden chipsticks so perhaps the symobolic value is worth it.