When does Alt Text Play during the Timeline?

I have several text boxes and images that would require considerable alt text for context. I need to know if I add alt text for e-readers, when does it get read? Does the e-reader read it at the time stamp int he timeline the object is placed? Will it stop the video's audio in order to read the alt text aloud and then pick up where it stopped or will it play both simultaneously? Or if I leave the text boxes and images as objects for the e-reader to recognize, will they become "viewable" by TABing on the keyboard and would this happen at the end of the video?

Context: I have a training with several long videos in it I am responsible for updating. Some errors and edits that are required are small and I do not have time to re-record several long videos including audio and visual elements. So, I have added to the timeline of some slides text boxes and corrected images to show in front of the video as it plays. I've synced when they appear and disappear for four different elements throughout one particular 9-minute video. Visually, the elements sync perfectly and are represented in the notes of the slide. The audio has not been changed, just edited to remove errors. The closed captioning and transcript will have the correct information in it, sans audio. My issue is for non-sighted learners listening to just the audio of the course, how do they get the information?

NOTE: There is an annual deadline for this training of December 31st. This is all just a temporary fix until I can re-record things next year. I know that re-recording the videos is going to be the only way to solve all these problems completely.