Soundboxes like this have been supplied by PayTM, BharatPe, Walmart PhonePe and other PSPs in India for over a year. I'm told that they have been supplied by AliPay and other PSPs to stores in China for many years.
I've been to dozens of stores that have a soundbox. While playing music is not a bad idea, I haven't seen a single store doing so.
The key functionality of the soundbox is to announce completion of a payment. Before soundbox came along, the store attendant had to stop what he was doing and look up his mobile phone to track the SMS or PUSH notif announcing payment completion. With the soundbox, he is able to multitask. Also, the SMS and PUSH go only to the phone used to sign up with the PSP. Quite often, that phone is with the storeowner, who may not be in the store all the time. As a result, the store attendant would need to look up the notification on the payor's phone. Crooks have been known to show a fabricated screenshot on their phone and scoot with the goods without paying. It was too late by the time the store realized that it had been swindled. Whereas the soundbox is a standalone device that's always present in the store.
Soundbox solves a very tangible pain area for stores, especially small stores where there are only one or two employees, there's no checkout line and the store attendant attends to the next customer by the time the previous customer makes the payment. Literacy or accessibility have primarily nothing to do with soundbox although they could be useful secondary use cases for it.