A "Loop Start" action can be used for this, E.G.
Start Loop While : Keyboard Key 'A' Is Pressed
Say, 'My text-to-speech phrase' (and wait until it completes)
End Loop
However, as the loop is synchronous, your command would not proceed until you let go of the key.
If that is undesirable, you'd need to run the loop in a separate command, and start that command asynchronously from the first command (using the "Execute Another Command" action with the "Wait until this command completes before continuing" option unchecked).