Right oh, here's my plan.
Toyed with the idea of having it all self contained in the M41-A. Even in my resin rifle, there wouldn't be enough space to fit parts to make it loud enough to hear over shouting, never mind the number of batteries required to get the amp up to 15V...
What I've opted for instead (and I did toy with the FM transmitter idea for a while!) is to run a female 3.5mm jack from the AspEng board, down the inside of my M41-A pistol grip. From there, a lead running down my arm, covered either by shirt sleeves or bandages and skin coloured surgical tape will run back to the amp, battery packs and speakers in my webbing.
This will enable me to have as big a battery pack, amp and speakers as will fit in my '58 pattern webbing, which should be quite a substantial setup.
I also figured out where that speaker came from; eBay, of all places. It was a truck radio speaker. I'll be taking something bigger into the AE with me...
As for the amp, I didn't use any preamp etc., just this amp:
http://www.maplin.co.uk/Module.aspx?Mod ... &doy=19m10
Works a treat at 15V with decent speakers.