The word "sisses" is not commonly used in everyday language, so there are few synonyms available. However, depending on context, possible synonyms include hisses, whispers, mumbles, murmurs, or rustles. Each of these words describes a soft, indistinct sound that might be produced by someone speaking in a low voice, or by something moving quietly. While they may not be exact replacements for "sisses," any of these words could be used to convey a similar sense of secret or muted communication. As with many words, the most appropriate synonym will depend on the specific context in which "sisses" is being used.