Ronster said:

This Removal of frequencies will only work providing what you are trying to remove does not clash with what you are trying to keep.

If the Dialogue is at the same frequency as a piece of background music then you have no chance of removing it.

The example shown in that video is fine because the police siren is nowhere near the frequency of the dialogue, but a full orchestral score elements of it will be at the same frequency as the dialogue so this won't fix the problem unless you have separate audio tracks.

I know that is an issue, but generally using the spectral analyze you can make more detailed selections. Unless, there something I don't know of, frequency removal is the only way through mono content assuming you are working only with the center channel.

It should be possible though since our brains can seperate the dialogue, music and sound effects from eachother. We just need to figure out how that works software wise.