What term refers to the mistaken substitution of one word for another word that sounds similar?

The term that describes the mistaken substitution of one word for another that sounds similar is indeed malapropism. This literary device often results in humorous effects, as the incorrect word chosen typically has a meaning that diverges sharply from the intended word, yet sounds somewhat alike.

Malapropisms can contribute to character development and comedic elements in literature. For example, a character might be portrayed as uneducated or foolish through the frequent use of malapropisms, thereby providing insight into their personality and often eliciting laughter from the audience.

The other terms do not fit this definition: alliteration refers to the repetition of initial consonant sounds in closely placed words, metonymy involves substituting the name of one thing with the name of something else closely associated with it, and synecdoche uses a part of something to represent the whole, or vice versa. Each of these serves different purposes in language and literature and does not pertain to the substitution mistake highlighted in the question.