Ok I managed to make it work (this will be my last post, I promise!). It's very important that you use the SND file that can be found on the latest uplodaded version of Metaaudio by Lagonauta (dated 19/12/2023). Using the SND file mentioned by user MRPIRATEFOX in the first post just did not work for me. It needs to be placed on the Half Life folder (where the .exe is located).