For each cinematic there is one SBT (SuBTitle) file found in Interface\Cinematics (interface.MPQ). It contains localized subtitles for movies.
At the beginning are 3 bytes 0xEF BB BF. Some kind of ID/Magic bytes/unknown stuff.
Each entry is made of a timestamp and a text. The entries are divided by an empty line.
00:00:04:06 - 00:00:13:01 Four years have passed since the mortal races banded together and stood united against the might of the Burning Legion. 00:00:14:00 - 00:00:22:05 Though Azeroth was saved, the tenuous pact between the Horde and the Alliance has all but evaporated. 00:00:24:06 - 00:00:28:21 The drums of war thunder once again.
The timestamp is hr:min:sec:msec.