It depends on the file, so my solution here won't guarantee a good result.
Anyway, first you need to download the video file and use VLC to play it.
On VLC menu, choose Tools > Efects and Filters. Then click on "Compressor" tab. You will see something like image below.
I know you will get overwhelmed with all the sliders, but you just need to take care of 2 things: Threshold and Makeup gain.
The step is,
1. While keeping the video file running, bring the Threshold slider down (to usually around 20~25) and see if all sounds are at the same levels (or close or at least the voice clear enough).
2. If everything is good just ignore the Makeup gain setting. Otherwise if the resultant audio is too low, increase Makeup gain slider slightly.
3. Click "Close" when finish and don't "Save" or else this setting will remain (which might not work on other video)
The point of doing all these is to make the sound of actors voice and BG music the same....so you could hear the voice even with loud music...there's no way to reduce volume of BG music separately though.