I have a Telefunken model TTV-210 TV. Picture is perfect but no sound - only noise and sometimes radio station. I have replaced everything related to sound - even swapped components with a working TV (same model). It seems to be a setup problem - cannot find sound settings in the menu.
Sounds like an internal problem. If the receiver works with external audio sources only, then the fault could be an electronic switch. Probe the interior ( carefully!) with a signal tracer around the speaker wires, including the external source inputs. Without identifying how the audio is switched, I can be of no further help.