Update 2018.09.22. Go to the website icm2018.org, and use the menu icon (three horizontal bars) to get a menu list that has ICM videos on it. That takes you to a YouTube channel which has some of the plenary and invited lecture videos being uploaded. If there is a problem accessing or playing the videos please let me know, and I will relay the information. Marcelo Viana says in an email that they are uploading the videos, but did not say when they would be done. I checked it out and saw some of the plenary lecture titles uploaded. Enjoy! End Update 2018.09.22.
I will ask the organizers tomorrow. However, I suspect from other things that the plenary and invited lectures will be given lower priority than the current output, as much is being done to popularize mathematics for Brazil. My guess is that Mark Sapir is right, and that you will have to wait a few days for the lectures meant for specialists (non-public). As a second guess, I imagine they are preparing Portuguese translations, and that doing so within ten days will be an impressive feat.
** Update 2018.08.04 **
Marcelo Viana graciously offered a moment of his time; he says he hopes (with a pessimistic estimate) that the end of month will see the videos posted. They have been working hard to make this Congress as paperless as possible (which on the whole is a good move, I think), and one of the tasks remaining is to update and post (a version of) the database of abstracts for use by the Congress members (and eventually the public, I imagine).
Gerhard "Impressive To Me, At Least" Paseman, 2018.08.03.