Title pretty much sums it up, I want to plug a speaker output (8 ohms boat anchor rig) into the microphone input of a laptop. This laptop has no "Line In" jack.
I assume it should be pretty easy with some sort of voltage divider network but don't know where to begin as far as values. No idea what the input impedance of the mike jack is.
Yes, a voltage divider will work fine.
Save yourself the math and find a 10K to 50K pot. A trimpot is best because it will be a set and forget situation, but a standard pot can be used as well.
Upper lug of pot to spkr output.
Lower lug of pot to the gnd/shield of both input and output.
Wiper of pot to Mic input.
Start with radio volume set for normal operation and this new pot set with wiper to gnd side and then start cracking the pot open until you get just enough audio to drive the thing and stop.
And don't listen to the crap that is all over the place regarding Impedance, its Audio. Rule of thumb with audio is that we can always drive a higher impedance input with a lower impedance source. Can't do it the other way around as attempting to drive a low impedance input with a high impedance source will load down the source. Plug and chug it with ohm's law should make it clear if you need to proof it, plugging and chugging a formula using several different realworld parameters is a very good way to learn and get a feel for such things.
And besides, the Mic input to computer sound is Low Impedance anyway. You are correct in thinking that all that is needed here is to lower the amplitude (AC voltage) of the signal to suit.