You can remove (kinda) the vocals from a song using audacity. The sound quality will not be extremely high quality in some cases but audacity is a free program. This can be used to make instrumental versions of songs to be included in your documentary.
Start with a new audacity file. Select File, then Import, and choose the song you’d like to remove the vocals from.
Select the down arrow next to the audio file name, select “Split Stereo Track”.
Select the down arrow on the first track, and change it to mono.
Click the second track, go to effects, then invert.
The last step is to change the bottom audio to mono. The vocals of the song are not removed completely, but this pushes them into the back and brings the background music to the front. It works best with songs that have strong vocals.