I was wondering if it was possible to besides changing the textures of a game/mod (texturepack) to change the specific sounds as well. So for example changing the wooden trapdoor into a metal textured one and then changing the sound so that it actually sound like a metal door slamming shut.
In the .minecraft\resources\ you will find a folder with newsounds you can make a backup of this and overwrite the .ogg files. You could use a program like audacity to edit or convert sounds to .ogg.
Once you overwrite some of these sound files in the minecraft folder you need to put this folder to read only. Otherwise MC will download the originals and overwrite them again. R-Mouse on the folder and select properties. All the way down you can select "read only". Or you could do this for every sound file separately.