Replacing a file is possible, it's just not useful for the creator or anyone else who is missing it. If you make any changes to a Var, don't share it, or use it as a dependency, it's only for your personal use.
The sound file should be packed with the Var. It's normally a .wav file. If you unpack the Var with the package manager and go into your Addonpackages folder, see if the file is actually there. If there is no sound file in the var, open the .json file in the saves folder of the unpacked var in a text editor. Search for ".wav", when you find it make a note of the exact file structure and name of the file. If you have the audio you want to add, make sure it is in the same file format (should be .wav) that the Var json calls for and rename it with exactly the same name. You then have to put it in exactly the same place. When you load the scene the var should find the file you added.
Here is an example. My latest scene has this line in the .json;
"url" : "SELF:/Custom/Sounds/Origin69/HLo/HLo1.wav"
If there was no Sounds folder in the unpacked var, you would need to add the Sounds,Origin69 and HLo folders and then add your renamed audio file in there. I just tested it and it does work for .wav files.