When watching the introduction of Inherit the Earth, the subtitles are shown when voices only was selected in the launcher for the game's option. For the rest of the game, the subtitle behaviour is correct.
This bug occurs in the English and German DOS talkie and is still present in the Daily Snapshot from May 19, 2012. This is probably because the strings were taken from the original EXE files and have now to be displayed manually.
Fixed in the latest daily version (commit 8c1ca65), thanks for reporting!