Adds an amplifying audio effect to an audio bus. Increases or decreases the volume being routed through the audio bus. $DOCS_URL/tutorials/audio/audio_buses.html Amount of amplification in decibels. Positive values make the sound louder, negative values make it quieter. Value can range from -80 to 24. Amount of amplification as a linear value. [b]Note:[/b] This member modifies [member volume_db] for convenience. The returned value is equivalent to the result of [method @GlobalScope.db_to_linear] on [member volume_db]. Setting this member is equivalent to setting [member volume_db] to the result of [method @GlobalScope.linear_to_db] on a value.