Actually I see
on every page there is a vertical threedotmenu just below the top left WGBH logo.
When I tap that it gives Celtic Soujourn"12.00 AM" as one of the options to play.
https://www.wgbh.org and the link there is playing the same programme.
Currently definitely the broadcast from 2 June 2018 -
:-) for me anyway, in the UK(~19.00 GMT 9th June)
Still don't see any way to go back\forward.
Or a play list.