How to add custom samples?

[BarbarianKabbage]

I want to add a different train horn sound (The one from Half Life 2) into OpenTTD. I cracked open the opensfx.obs but it led to the sample.cat file. Can somebody please explain how to add or replace samples into a .cat file?

[Eddi]

you can add sounds in a .grf file with the sound_effect callback

also, there is catcodec to handle the sample.cat file. however, you can only replace sounds that way, not add new ones.