Instead, we use other adverbs to convey different degrees of softness:
Comparative:
* More softly
* Softer (used in a more metaphorical sense, referring to a more gentle sound or touch)
Superlative:
* Most softly
* Softest (again, used more metaphorically)
For example:
* He spoke softly to her. (original form)
* He spoke more softly to her than he had before. (comparative)
* He whispered softly in her ear. (metaphorical comparative)
* The music played most softly in the background. (superlative)
* The night air felt softest in the garden. (metaphorical superlative)