I found a fairly easy way round this. There is a second audio output available on the rear of the radio which has a 200mV output level, that is independent of the Volume control on the front panel. This is on a stereo 3.5mm jack socket marked Data In/Out. It is intended for connecting a packet TNC, or other data-mode device to the radio, but the audio quality from this socket is good.
Red arrow shows the Data In/Out connector on the back of the radio.
The headphone volume is controlled by the AF gain control on the front of the rig and the PC speakers are controlled with their own volume control.
One of the criticisms of the FT847 is the "steppy" digital AF gain control on the rig, making it hard to get the right listening level - so using an external speaker on this audio output might be a way round this in the shack - so this adaptor might be useful for more than the occasional special event station.
Happy Operating, and 73