IIRC, the problem with historical music was trademark, as in, Paradox can't sell games with possibly still-trademarked musical pieces in them. So, I propose we put together a free mod that gives every major (so ENG, FRA, ITA, GER, SOV, JAP, USA) its own little soundtrack. Why majors? They're the easiest.
Anyways, my little contribution...SOV's war track, courtesy of the Red Army Choir!
My personal favorite is We Are The Red Cavalry.
We only need 10~ songs at the most for each track, so 10*2*7 = 140, 20 songs (10 war, 10 peace) for each major, with seven majors. There are definitely collections around on the internet (I remember one with Koniggratzer Marsch, Preussens Gloria, Es war ein Edelweiss, Panzerlied, and Die Wacht Am Rhein, among others), so we only need to find a few of these collections (like the one I found above) and we're set.