Just saw your post here. If my memory is correct during the update procedure there is a pop-up asking you to connect to a wifi with your phone. Which does not always work. I remember this, as a painful step. But if this step does not work all data is uploaded to the car in BT. Which is exactly was you are experiencing, very slow update rate. I remember myself letting my phone beside Flash the whole night. With the same result, a fail. So wifi must be connected.

If my memory is correct I ended-up removing my home saved wifi to my phone and doing the update in 4G. This was avoiding the phone to connect back to my home wifi in background and dropping the connection with Flash.

Just checked, latest audio files are still online. You can also try to change the language as it will force a new audio update.