Questions & Answers about Spæjarinn heyrir þá tala um sönnunargagnið.
Why is there no að before the verb tala?
Because heyra (to hear) is a perception verb. When you see or hear someone doing something, Icelandic uses the Accusative with Infinitive construction. Just like in English ("hears them talk", not "hears them to talk"), you drop the infinitive marker að and use the bare infinitive tala.
Why do we use þá instead of þeir for the people talking?
In this structure, the people doing the talking are grammatically the direct object of the main verb heyrir. Because heyra takes the accusative case, the nominative þeir (they) becomes the accusative þá (them).
Why is sönnunargagnið in the accusative case?
The preposition um controls the case here. When um means "about", it always forces the following noun into the accusative case. Because sönnunargagn is a neuter word, its accusative definite form happens to look exactly like its nominative form: sönnunargagnið.