Is there a literary term for when an author, such as Jane Austen of Sense and Sensibility, chooses not to indicate who is speaking a line of dialogue, but instead the reader knows who is speaking from the preceding dialogue?
For example, Austen chooses not to identify Colonel Brandon as the speaker when he says, "I cannot afford to lose one hour." The effect of this lack of "who said it" is to convey the silent astonishment of the surrounding party and also the urgency of Brandon's claim, but what is the technique called?
The information an author gives either before or after a line of dialogue in order to identify the speaker is called a dialogue tag. However, as you say, when it is already obvious as to who the speaker is, an author may choose not to give a dialogue tag. We simply call this omitting the dialogue tag. This can often work whenever there is a long exchange between two characters. Dialogue tags have a tendency to draw a reader away from the story; therefore, omitting the tags can produce the effect of moving the dialogue along faster, drawing the reader deeper into the dialogue, and creating more emotional impact through more realism.
There are certainly several places in this scene in which Austen chose to omit the dialogue tags. The first incident occurs after Colonel Brandon returns to the breakfast room after reading his very important, newly arrived letter. Here, Mrs. Jennings enters an exchange with him in which she rudely tries to fish out who the letter is from, even going so far as to say, "Come, come, this won't do, Colonel; so let us hear the truth of it" (Ch. 13). This rapid fire exchange between Colonel Brandon and Mrs. Jennings is broken when Mrs. Jennings' daughter, Lady Middleton, scolds her mother for being so rude and nosy. From there, the dialogue continues into a debate with the other characters as to whether or not he should stay for the party or allow his urgent business to take him immediately to London. When Colonel Brandon says the line in question, "I wish it could be so easily settled. But it is not in my power to delay my journey for one day!," Austen was able to omit the dialogue tag because by that point, the reader is well acquainted with Brandon's problem. The reader knows his letter has him very upset and knows that he must immediately go into London, abandoning the party he had arranged.
Since Austen was able to omit the dialogue tag from this line, the reader is not interrupted from either the flow of the story or the sense of Brandon's emotions. Not only does the reader very clearly hear Brandon's voice as the speaker of these lines, the reader also very clearly hears just how distressed Brandon is and is therefore able to empathize with him when none of the other characters are able to understand.