Common:
* Quietly
* Quietude (state of being quiet)
Less common:
* Quiet-spoken (speaking in a soft voice)
* Quiet-living (living a peaceful life)
* Quiet-mannered (having a calm and reserved demeanor)
* Quiet-voiced (having a soft voice)
* Quiet-toned (having a soft and subdued tone)
You could also create your own compound words! For example:
* Quiet-time (a period of time for quiet reflection)
* Quiet-space (a place for peace and tranquility)
* Quiet-joy (a deep and peaceful kind of happiness)
Let me know if you'd like me to explore any of these ideas further!