1. You can either put the OGG/FLAC/etc. files into the original SOUND subfolder (in the UFO folder).
2. Or you can package them as mods, with the following structure:
<openxcom user folder>/mods/YourModName
<openxcom user folder>/mods/YourModName/metadata.yml
<openxcom user folder>/mods/YourModName/SOUND/<put all sound files here>