I thought problems with radio streaming manifested themselves as dropouts or a complete 'hang' (silence) rather than music slowing down.
Given that all BBC radio music is fed by digital servers (no turntables or tapes to go slow), then I am a bit baffled by your description of the fault (like someone slowing down a record player).
What are you using to play the stream? There was something on the Sonos forum the other day about an internet stream running at half speed and it was something to do with the codec of the stream not being fully supported by the player (there's two versions of the codec and the player only supported the lower rate version and played the higher rate one at half speed or something). I think that was the BBC as well.
Perhaps check if there's any updates for the player or try something else to listen to it with (unless you're using iPlayer?).