What is a single word alternative to "whispered"?

Solution 1:

You're going the wrong way about it.

Use whisper for the neutral whispering—like you just did in your own explanation of what you were asking about—where the most important thing is only the volume of the voice.

Use any of a myriad of other verbs—hiss (anger), gasp (shock or fear), murmur & mutter (uncertain), sigh (exhaustion or caring), purr (arousal), breathe & pant (fear), &c. &c. &c.—to add nuance as needed in places where that shading is more important. Even with words like confide that don't technically involve loudness, some delicacy is implicit in the idea. Where necessary, you can use adverbs for their volume or just add more context: He leaned in and... He felt her warm breath against his neck as she... Making sure El Guapo could not hear, they...