When nothing speaks
Silence has a few different causes, and what the panel does tells you which one you have before you change anything.
You selected some text, you tapped the key, and nothing happened. Silence is not one problem. It is a few, and Chippy behaves differently for each, so start with what the panel did.
Nothing appeared at all
No panel, no sentence, nothing. The key never reached Chippy, and there are two reasons for that. They look identical from where you are sitting, so do not guess between them: open Chippy's menu in the menu bar and look.
If the permission has been turned off, the walk is waiting there at its permission card, and the button on it opens the Accessibility row of System Settings. Without Accessibility, Chippy cannot ask another app what you selected, so it gives up before it asks.
If the menu looks normal, it is the key. It is the Option key to the right of the space bar, tapped on its own and released, and the left one does nothing. A hold with any other key is that shortcut, and Chippy stays out of it deliberately.
The panel appeared and said there is nothing to speak
Chippy heard the key and asked, and the answer came back empty, which means the app in front did not hand over a selection.
Chippy asks for it the way Command C does and waits a fifth of a second. So anything you cannot copy, Chippy cannot read: a picture of text, a scanned page with no text layer, a field that blocks copying. Press Command C on that selection yourself. If nothing lands on your clipboard, Chippy was asking for something that was not there.
There is a way through. Copy what you want with Command C, then tap the key with nothing selected. Chippy reads what you copied, and the panel says it came from the clipboard.
The panel appeared and said Chippy could not speak that
Your text arrived and the voice did not. Try again in a moment.
If the sentence is about signing in, verifying your email or the week's listening, that is a different page: Your weekly free listening.
The words are moving and you hear nothing
If the panel is lit and stepping through the words, the read is working and only the sound is missing. Check Chippy's own volume in the panel before your Mac's: at zero it mutes the recording while everything else carries on exactly as it should.
During the first run
While the walk is on its Your turn card, a selection from another window is refused on purpose and says so. Use the words on the card. The rest of your Mac comes after the walk.