General:
* Quiet
* Peaceful
* Calm
* Serene
* Tranquil
* Harmonious
* Subdued
* Mellow
* Soothing
* Gentle
Specific to sound:
* Soft
* Muffled
* Melodious
* Sweet
* Harmonious
The best antonym will depend on the specific context in which you're using the word "raucous."