一個耳報 A Voice Behind the Ear
孫大聖，他卻變得輕巧，在耳根後，若像一個耳報；但他說話，惟三藏聽見，別人不聞。孙大圣，他却变得轻巧，在耳根后，若像一个耳报；但他说话，惟三藏听见，别人不闻。
逐句解 Line by line
The Great Sage Sun had made his change a nimble one: he was behind the root of the ear, like an ear-reporter;
Here 得 is de, the light syllable hooking the verb to a verdict on how it was done. An 耳報 is an ear-report, the vernacular for an informer who whispers what he has heard — and also, in popular belief, for a small familiar spirit kept about the person that tells its master what is going on. Pilgrim has become one.
so that when he spoke, only Tripitaka heard it and nobody else caught a word.
The mechanics are stated flatly and they are the reason the whole scene works. Two characters can now hold a private conversation in the middle of a banquet. 惟 is the bookish only, which gives the sentence a slightly formal finish.
The novel pauses to explain a piece of stagecraft, which it does not often do. It matters here because everything that follows depends on it: Pilgrim can speak and only his master can hear.
The ear-reporter is a nice piece of popular belief. Folk tradition held that certain adepts kept a small spirit near the ear that carried news to them, and the word came to mean any whispering informant. Pilgrim, who is neither a spirit familiar nor an informer but is briefly doing the work of both, gets described with the ordinary word for it.