BIG GRAIN OF SALT HERE. Not a Mac user but watched a few tutorials on this. There's some guessing here...obviously you going to need to know how to change controls using voiceover and have the voiceover also tell you what's on the screen. Hopefully this is doable?
It seems that either you hid the Favorites or you removed the All Inboxes from Favorites.
Here's how we tell: Under Favorites, there is typically stuff like 'All Inboxes', 'All Sent', 'All Drafts'.
1) If the other stuff in Favorites is gone too, then it seems likely you just hid Favorites? To the right of the Favorites control, there is a small arrow control that transitions from V to > to the right of Favorites. Clicking that control should restore the Favorites, which has All Inboxes. In theory.
2) If you DO have other Favorites, then maybe you removed the All Inboxes from it. There is a small + control to the right of Favorites. Click on that control. A new window will appear, go to it. That window has a control marked Mailbox To Add. Click on it. It will bring up a dropdown with items you can add. The top item is All Inboxes. Choose that. Beneath the Mailbox to Add controls are Cancel and OK buttons. Click OK. This should readd it to your favorites.
mightyweasel