Today's popular container formats, such as Matroska (MKV), can hold multiple audio tracks, usually for different languages. It can be the case that the first track isn't English, which means you'll have to constantly change it over, especially if you've queued multiple videos. If you're using VLC, there is a way to fix this.
There's isn't a drop-down menu to select from to force the English track to be used by default — you'll have to enter it manually. To do this, fire up VLC, go to "Tools -> Preferences" and select the "Audio" button.
At the bottom of this window, you'll see a section called "Tracks" and within that, a textbox labelled "Preferred audio language". You'll want to type in "EN" or "ENG" (lowercase is fine too). If you'd prefer a different language, a full list of codes is available here.
It goes without saying that, for this to work, the audio track has to exist within the container file and be properly flagged.