Hi Rich0,
Good to see another musician in here
If I've understood you right, you've got: bass drum -> mic -> mixer -> strobe.
hmm, why the mixer? Unless this isn't for live, but a pre-recorded song/tune.
And also, generally a mixer has multiple inputs, mixes them, and sends the conbiantion out, so maybe other instruments would trigger the strobe (like a dirty guitar riff in drop C). Anyway, assuming you've just got the kick drum coming out of your mixer, linelevel, I would suggest a simple potentiometer, acting as a voltage divider. One lug to the mixer signal, the other to ground, and the wiper straight into the strobe trigger. That way you can fiddle with the pot until you are satisfied that the strobe triggers nicely on the 'kick'. It may work fine if you just connect it up directly. Line-level is quite a bit more than mic level, but you're not listening to it, so any clipping of the input shouldn't really be a problem, it'll just trigger the strobe on very quiet sounds, possibly making it too sensitive.
If however, you've got loads of instruments coming out of your mixer, and you still want just the kick drum to trigger the strobe, then you would need a low pass filter, to get rid of all the instruments above.
Depending on how well versed you are with electronics, a good way to control the level at which the strobe triggers, is to use a comparator. This will produce a single pulse whenever your input (from your mixer) goes above a certain level, which you vary, and its alot neater than a voltage divider.
So, just to cut to the chase. Yes, I would say removing the mic from the PCB and replacing it with something designed for line level should work. Some PCB mic's are powered on the same connector as the signal, so, first thing you'll need to connect to the pad where the MIC was (signal) is a capacitor, say, 10uf. That'll block any DC from the PCB going into any circuit you are planning to use. Then the rest is up to you, just experiment, I don't think you can blow the strobe trigger, and theres no point in building a nice neat comparator circuit if a potentiometer (as a voltage divider) works fine.
Sorry for the lost reply/giberish, but I don't like being vague about things. If I've completely missed to point of what you're asking, sorry, its late
Good luck,
Buried(in)Code