Although some of the default collision sound effects are quite good, the most commonly heard sound (caused by avatars bumping into wood) has a resonant quality that doesn't occur when a shoe bumps into a wall in real life.
Since it is heard so often, updating this one sound effect would have an outsized impact on the qualitative experience of using SL for all users.
This is not a request for the default material to be changed away from wood. Although metal might make more sense for sci-fi builds, and I personally use stone (as Firestorm allows overriding it), wood has always been a reasonable neutral starting point.
This is also not a request for viewer overrides for the default collision sounds—there are quite a few and the UI would be a lot of work for something so obscure.
I know how to use llCollisionSound().
I just really think the default needs to be improved, as it is ancient but prominent.