I assume that you have attached it via SCART cable...since SCART plugs have 21 pins, and carry video, audio and switching signals ,sometimes a bad connection on either TV side or Top Box side ,the sound goes off first..Make sure your SCART plugs securely fit to either side.
If it does not help pls visit the URL below for further advise before going to blame the Top Box....Hope this helps! Take care and please Remember to rate/vote and give me
4 Thumbs Up for me to continue for Helping out the Community :)
Set-top Box Connection Advisor
This page is designed to help you with connecting your
Freeview, BT Vision, Virgin Cable, Freesat or Sky satellite set-top box
to your home TV system. http://www.radioandtelly.co.uk/connecthelp.php