This is a modding tool for Doom Eternal and it enables audio modifications to the game. All the game audio is stored in a bunch of archives in the .snd format (eg. sfx.snd, music.snd, vo_English(US).snd,...). Inside those, there are embedded audio files in either Wwise's WEM format (used for music) or Opus (used for sound effects). This tool allows you to inject your own custom WEM and Opus files into the archive and use them as a replacement for pre-existing ones. That way you can replace any in-game sound for your own.
To use this program you need to know the internal ID of the audio file you want to replace. These IDs are located at the top of the SND archive. To start this program, run:
SND_Inject.exe [path_to_SND_archive] [SFX_ID] [path_to_your_audio_file]
Supported extensions for audio files are .wem, .opus and .ogg (which is an alternate extension for Opus files). For Opus files to work, you also need to have the file opusdec.exe (Opus Decoder) in the same folder as the executable of SND Inject.
Changes to the SND file cannot be reversed with this software so I suggest you create a backup before injecting your WEM files.