Grundig Music Boy 70 - loss of audio

Hi, I have a Grundig Music Boy 70 portable radio with very faint audio output.
The speaker, 3.2 ohms, works fine, the volume control works fine. Replaced various suspect electrolytic capacitors which didn't help.
On power up, the initial volume is louder but it quickly fades to almost zero, in less than a minute.
Been looking for schematic diagram without much luck.
There are two main ICs, a TA2132BPG (believe as pre-amp stage) and a LS820, 8pin, (probably audio amp) but, had little luck on data sheets for those, too..
Any tips on troubleshooting this problem?
I would like to save this radio from a destiny in the landfill.

Much appreciated,
Blade.