The Curtiss C-46 uses the Pratt & Whitney R-2800 Double Wasp (same as the DC-6 used a.o.) where the B-25 Mitchell is powered by the Wright R-2600 Twin Cyclone which, if you ask me, sounds very different compared to the P&W R-2800 (i used to think that during fly-by the B-25 sounded very much like a 'Harley-Davidson on the fly' while clearly not so much true for the DC-6 or other R-2800 equipped aircraft..)
Since the C-46 was developed by the Curtiss-Wright company you'd wonder why they went for the P&W R-2800 instead of 'their own' Wright R-2600. Maybe more power ?...
As for the Maam-Sim B-25 soundsuite, we developed it ourself by recording the engine sound of MAAM's own B-25 'Briefing Time', both inside and out. Inadvertedly it possibly became one of the most expensive sound suites ever created for FS. Although ages ago I feel i still am not at liberty to tell you anything more about that...
As for the C-46 engine soundsuite. If you ask me you'd get the most satisfying result by using Ted 'Tufun' Wolfgang's amazing engine sound for Manfred's freeware C-47. Original for the FS9 version, can't remember if it made it as far as FSX but i do think so. Later it became even superseded by a C-47 soundsuite created by a gentleman who's name i can't recall atm.
I still think both these C-47 engine soundsuites are some of the very best twin prop engine soundsuites ever created for FS9 and FSX/P3D. Like the double row of cylinders of the P&W R-1830 that powered the C-47/DC-3, these 2 superb engine soundsuites doubled the fun it already was to fly Manfred's likewise superb C-47 in FS9 and FSX/P3D. I'd almost say "those were the days my friend, we thought they'd never end.."