With Mojang dropping support for the basic sound files for Tekkit Classic here is a bit of info you will need in order to install the resource pack
If you're having the issue of vanilla minecraft sounds not working on tekkit classic, here is how to fix it:
1 - Download this file which contains the sound files you're missing.
2 - Unzip the folder to somewhere easily accessible, for example the Desktop.
3 - Locate your resources folder in tekkit at %appdata% > .technic > modpacks > tekkit.
4 - Delete the folder resources and replace it with the one you have just downloaded.
5 - Boot up tekkit again and your sounds should be back.
If you're having the issue of vanilla minecraft sounds not working on tekkit classic, here is how to fix it:
1 - Download this file which contains the sound files you're missing.
2 - Unzip the folder to somewhere easily accessible, for example the Desktop.
3 - Locate your resources folder in tekkit at %appdata% > .technic > modpacks > tekkit.
4 - Delete the folder resources and replace it with the one you have just downloaded.
5 - Boot up tekkit again and your sounds should be back.