Ok, let's manually install the latest navcore for your Tomtom to get around this chicken-egg situation. Download from here:
Back up what you now have on your SD card.
Delete all files and folders on the SD card except the map folder.
Extract the contents of this cab to your SD card and restart your Go with it. Now try TTHome 2.7 with it and have it update secondary files like voices. If you still miss voices or other files, copy them back from your backup.
You need to patch the navcore before using it with a downloaded map.