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Paul, what you are experiencing would unfortunately indicate an internal hardware fault, namely that the amplifier circuitry has failed. This isn't surprising to see as these systems age and components like capacitors start to fail, though it could just as easily be the result of damage from a power surge.
Other than being muted or volume down, there is no other setting or external cause of this, it can really only be a hardware defect. Theheadphone jack works because it is a preamp signal, which is also why it isn't very loud.
There is one step you can take here, both as a possible fix, as well as a diagnostic evaluation 1) Unplug the power cord from the unit at the mains
2) Wait at least 1 hour (60+ Minutes)
3) Press and hold the power button (on the set, not the remote) for 60 seconds 4) Keep holding power button while plugging back in, and for another 60 seconds after. When plugging back in, go directly to the wall outlet, bypassing any power strips or surge protectors.
If this reset process does not work, it would unfortunately confirm an internal hardware failure.
At that point, you would be looking at something that would require repairs, though this simply wouldn't be practical. This model is long discontinued, and shelf systems like this have no parts or support available, they are essentially built to be disposable. It would cost more in diagnostic fees just to have an engineer look at it than it would to buy a whole new system.
As an alternative though, what you could do is to plug in an external sound system to it. For £25 or less you can get a nice pair of powered (active) computer speakers, which would simply plug to the headphone jack on the philips system. This would give you sound back through the external speakers, while retaining the functions of the main unit