I am not aware of any way to specifically reduce the volume of the sound effect for the opening of gift boxes. You can, however, adjust the volume for "sound effects" separately from music and commentator's volume.
To change volume on Special Effects, Music or Commentator you should got to SETTINGS (this may appear as a cog wheel or gear in the bottom right of your screen next to the EXIT icon), from Settings, you can see the AUDIO adjustments and from there it is evident what to do.
I hope this helps.