You are likely playing a 5.1 track through a 2.0 (stereo) system. The center channel (where dialogue lives) is being lost. Go to your audio settings and enable "Stereo Downmix" or "Night Mode." Alternatively, in VLC, go to Audio > Audio Device > Stereo .

The technical quality of the English track varies slightly depending on how you are viewing the film. Audio Track Specification Key Features DTS-HD Master Audio 5.1

The original English audio is often accompanied by English SDH (Subtitles for the Deaf and Hard of Hearing), which are highly recommended due to the fast pace and overlapping dialogue.

If you are learning English, The Wolf of Wall Street is a terrible place to start (it is advanced slang and slurring). However, if you are advanced and want to learn: