I recommend cloning the asset you want to change if you haven't already and then open up the "new asset" as this will be called in Explorer and not just to edit the config.txt file which you will need to anyway.
You can then examine the file format for your sound files using an audio editor such as Audacity or whatever else you will want to use. This will give you the bit rate information and other properties you need to create your own audio. When you do create your own audio, use the same file name as the original and overwrite that file.
Once you have that, you can edit the config.txt file and replace the Username with one that makes sense, so you know which freight car is yours which one is the original. Keep in mind when editing the config.txt file, you need to ensure that the formatting is correct, otherwise the asset will end up faulty.
After your edits, close Windows File Explorer, close the config.txt file, and submit the asset.
