There are many synonyms for the phrase "speak lower." Some of the most common include whisper, murmur, mumble, mutter, and hush. When someone speaks lower, they are generally trying to be quiet or keep their voice down. This could be because they don't want to disturb others, because they're in a public place where loud talking isn't appropriate, or because they're trying to communicate without being overheard. Whatever the reason, choosing the right synonym for "speak lower" can help to convey the right tone and meaning. So whether you're whispering, murmuring, or muttering, make sure that you're speaking at the right volume for the situation at hand.