This is actually in response to the Author's notes at the end of the chapter.
Yes, you can say things quietly without muttering, murmuring or whispering. In fact using "quietly" or "softly" is recommended if you're trying to convey that the individual speaking is carrying on a normal conversation, just at lower volume.
Muttering, whispering and murmuring all have connotations that don't send the same message as "spoke softly" or "said quietly"