Where can we find out whose voice is on the checkout machines at Wal-Mart?
You’d have to find the production company that got the contract from the manufacturer. They would be the one who hired the voice talent.
You might try to Google “Voice Talent Walmart (or the name of the machine manufacturer)” and see if the voice talent put a line on their site about doing the Walmart gig. I know I would.
The self service checkouts I’ve used are made by National Cash Register (NCR). The Walmart near me recently removed those units (perhaps for security reasons) but I do recall they used a female voice.