I found the book where I originally saw the mention of loudspeakers on trucks, telling people to turn in their gold and other valuables for the new currency, "gold yuans", which became just as worthless as the previous yuans within a short period of time.

It's in Last Boat Out Of Shanghai, by Helen Zia. It's a compilation of stories from various refugees from China, all coming from different circumstances. Super read, I'm going through it again.