דבר dbr in the piel is the most commonly used word for "speak": Am 3:1, 8 the qal has much the same sense: Am 5:10.
 
 dabar "word, message, story or business/affair". The usage in Am 1:1 is discussed in the commentary.

In the prophets the dabar is often the prophet's oracle 3:1; 4:1; 5:1; 7:10 (cf. 7:16; 8:11, 12).

In Amos 3:7 "thing" seems best to describe what God is doing and announcing.

The phrase לֹא דָבָר lo' dabar in Am 6:13 can either mean "nothing" ('no thing') or refer to a town east Jordan (2 Sam 9:4-5; 17:27).

 At 1:1, I have argued that we should render dibre as "story" (see Andersen & Freedman, 184-185 - though their translation does not reflect their discussion).