Connect 8 ohm speaker output to microphone input on another system
If you are connecting a speaker output to a microphone input, I would be very worried with connecting the ground from the speaker to the microphone. As microphone inputs are very keen to pick up noise and ground loops are among the worst creators of noise, I would rather carry the signal differentially. I'd use a transformer to pick up the signal from the speaker (pins "Line out" and "Line Gnd" in your schematic should be good points for this) and then carry the signal inside a shielded cable to the microphone.

If you need to do this regularly "in the field", there's someting called a "Leem Box" or a "Leem Direct-Box" which might be a helpful component in various situations of this kind.