The correct answer is 1. His manner of speaking changes rapidly.
On the previous sentence the author describes Usher's changing attitudes and does the same with his way of speaking. Although he describes Usher's manner of speaking as weighty, unhurried, hollow sounding, leaden and guttural, the author states that Usher can quickly change from a trembling to an energetic tone.
This text specifically is about how the mannerisms of Usher are unstable and changing. Most likely because his sanity is not intact, he can speak energetically and suddenly pull out a slow and guttural sound out of his throat.
