Sometimes language surprises me in tiny ways: 'murmur' and 'whisper' both mean quiet speech, but they live in different neighborhoods of meaning. When I read a novel or watch a scene unfold, I reach for 'murmur' whenever the sound feels diffuse, ongoing, or collective. For example, in a crowded room a single person's low voice creates a 'whisper', but the ripple of low, indistinct voices across the audience becomes a 'murmur'.
I also use 'murmur' for non-human soft sounds—the stream that 'murmurs' under the bridge, or the wind that 'murmurs' through winter branches. Those images are quieter and more atmospheric than the intimate secrecy that 'whisper' implies. In journalism or political writing you'll see 'murmur' used for low-level dissent: 'a murmur of discontent spread through the crowd' sounds more social and less conspiratorial than 'a whisper of discontent'.
Finally, don't forget technical contexts: 'murmur' turns up in medicine (a heart murmur) and in stage directions or poetry to suggest texture rather than a discrete speech act. I tend to pick 'murmur' when I want a soft background feeling, a continuous hum of voices or nature—it's moodier and more atmospheric than a secretive whisper, and I love how it colors a scene differently.