OMG!!! I really did not expect that to work!!! But yeah just saying "USB" while pressing the right voice button, and it resumed playing the track I was listening to from before! It's not worked on one occasion - but the other three or four times it worked!
Now just need Tesla to add more voice commands. Would like "skip back xx seconds" / "skip forward xx seconds" + "skip back/forward xx minutes" . Would be really useful for podcasts and audiobooks - especially if you've missed a bit due to the satnav chatting away or needing to extra-concentrate on some driving! Some of my audio books and podcasts are 1 hour long and fast-forwarding / rewinding through them is a pain and impossible to do while driving!