One of the reviews linked above says the person loaded some music into the empty area of the card to play. And elsewhere two of the reviews speak in less detail of loading music to other cards to play. So it may work.
We need a buyer to report in, but those people are not likely to be the experiment-and-report type, which is why they bought the device. Maybe a relative will do it for us.
Just yesterday have seen the ad showing a gal with one of the SlotRadio. All other ads I have seen have no hands to show the scale. I had impression this thing was much larger than it is. It is very cute.