2. Specify a different audio source in your Audio()/AudioWait() sequencer command. That audio source can have a different Volume setting or be routed to a different audio mixer group with a lower volume.
3. Write a custom sequencer command -- for example, a subclass of SequencerCommandAudioWait -- in which you can specify the volume to use for the audio clip.
4. Maybe easier: Write a custom sequencer command that sets the volume of an audio source. You can use this before calling Audio()/AudioWait() to set the volume to what you want.
5. Use a third party audio manager such as Master Audio, FMOD, or Wwise, all of which the Dialogue System can integrate with.