volume attribute
This attribute specifies the speaking volume for the utterance. It ranges between 0 and 1 inclusive, with 0 being the lowest volume and 1 the highest volume, with a default of 1. If SSML is used, this value will be overridden by prosody tags in the markup.
Click to execute window.speechSynthesis.speak() with volume attribute set to 0, 0.2, 0.4, 0.6, 1.

Manaul Test:

Does the volume of audio output change?