If you're talking about volume:
* Quieter: This is the most common opposite.
* Softer: This is similar to quieter, but often implies a more gradual decrease in volume.
* Silent: This means no sound at all.
If you're talking about intensity:
* Fainter: This describes something less intense or noticeable.
* Milder: This implies a gentler or less forceful approach.
* Subdued: This means quiet and restrained.
If you're talking about something being more noticeable:
* Less noticeable: This means it's harder to see or hear.
* More subtle: This means it's less obvious and more delicate.
The best opposite for "louder" will depend on the specific situation and what you're trying to convey.