My DTR-T2100 also recently became a model with silent radio playback. There has been a suggestion from a Youview representative that disconnecting the HDMI lead and reconnecting can occasionally bring back the sound. This does not work with me.
What works for me is connecting to the TV using a SCART cable and also disconnecting the HDMI lead.
RCA red and white leads to an amplifier should also work when the HDMI is disconnected, but isn't very practical as you would lose the ability to see what you are playing unless you also added a SCART lead.
Optical out has the same issue withe sound being muted and presumably has the same work round of temporarily removing the HDMI connection.
If you start playback of a radio recording and get no sound then with the recording still playing press the ON/OFF button to put the box into standby then press the ON/OFF button again and you should hopefully find the radio recording still playing but this time with sound.