Sound should always be opt-in due to browser autoplay restrictions and user preference. The best implementations offer clear audio controls, use sound to reinforce interactions rather than distract from them, and provide a fully functional experience when muted.
Music artist sites, immersive brand experiences, interactive storytelling, art installations, and portfolio pieces benefit most. Sound adds emotional depth and memorability to these experiences, but must be implemented thoughtfully with clear mute controls and reduced-volume defaults.