I have a couple of Vizios (older arc connection, and new w/ earc) that always turn on via voice control to the last used hdmi input (apple tv). I’m not sure that your issue would be caused by a tv setting. I’d check that my hdmi cables were connected to the correct ports on the TV (Arc into arc port).  I don’t own a fire stick, but I’d start with the power on behavior, and hope that solves the other.

I don’t use voice control to turn on my Vizios, both are connected to Apple TV’s….and sometimes, when they’re turned on by CEC, they go to the silly Vizio ‘smart TV’ page, rather than the input I was using when the TV was turned off (also by CEC). Which is frustrating, and I bet is something Vizio causes to happen. 
 

Not quite the same thing, but I’d start asking Vizio about that issue, it’s their interpretation of the command that’s failing, not Sonos’. 

Vizio did a software update that caused the TVs to always turn on to the vizio streaming input. I never saw mine do that, maybe because that one  was usually switched on via HomeKit, which switches the input to my Apple TV. If you’re still having an issue with that, they recently added a toggle in tv settings to have it not default to their streaming os/input.

Thanks for this info. I will look for this in tv settings and see if it does anything.

And thanks everyone for the replies. None of this is super important, I don’t often use these voice commands but it is weird that this is now happening since I hooked up the Sonos Arc. 

Prior to the Arc, I used a traditional AV receiver and an XBox and voice commands worked exactly as expected with Firestick, with XBox and with Alexa. I know everything is connected properly, the TV is 10 years old, however, so some oddities are not surprising.