SARAH:  Gertrude continues to ask what exactly she's done that has made Hamlet raise his voice to her.

RALPH:  When she says "in the index" she means the list of metaphorical images that he's just gotten through describing — as if it were an index or a table of contents.

SARAH:  She is also punning on the word act.  As we well know from studying Shakespeare, his plays are divided into acts, usually five of them.  And Gertrude is suggesting that this act is being preceded by a loud index, where index would mean a prelude or an introduction to what follows.