Parece que falta a ferramenta sox
installation
sudo apt-get install sox
deve corrigir isso
Edit: Instalando sozinho não corrigiu, você tem que dizer r128gain onde procurar pela biblioteca sox
mv /home/username/r128gain-1.0.11-linux64-gtk3/r128gain-tools/libsox.so.2 /home/username/r128gain-1.0.11-linux64-gtk3/r128gain-tools/libsox.so.2_backup
ln -s /usr/lib/x86_64-linux-gnu/libsox.so.2 /home/username/r128gain-1.0.11-linux64-gtk3/r128gain-tools/libsox.so.2
Então funciona
./r128gain /usr/share/sounds/alsa/Noise.wavSoX sucessfully loaded.
FFmpeg sucessfully loaded.
analyzing ...
[1/1] "Noise.wav": -29.7 LUFS (6.7 LU)
peak: -17.9 TPFS, range: 0.0 LU
[ALBUM]: -29.7 LUFS (6.7 LU)
peak: -17.9 TPFS, range: 0.0 LU
done.