Nice point Delphine:
I imagine as much work would go into the heist as blame shifting so that the network didn't have to take responsibility for the 'action.'

With sufficiently professional broadcasts, the difference between the original and the injected news may be significantly diminished, leading to the people not being able to distinguish between real and fake information. (Like if they were now. *cough*)

A person able to immitate other people's voices may wreak quite a havoc into a radio transmission, impersonating a newscaster or a figure of authority.

Another use in voice immitation is for e.g. CB or police transceivers, impersonating people with an authority and injecting commands (or at least causing confusion).

If accessing broadcasts is so "easy", hundreds of attacks directed simultaneously on such breaches could allow a group to take over the entire global information network and broacast fake news or videos to control opinions and minds... A group of dissidents or anarchists could, for example, broadcast images that would make people join their cause or that would discredit governments.

I think that would be very interesting!

But of course, what would also be interesting is to see, how such a group would prepare a global attack. I am sure it would require months - if not years - of preparation to be ready. Taking the control of the news channels, radio waves and the internet at the same time would be like the apocalypse of the Industrialised World...