The M button should light up red for it to function; there is no sound. Your assumption that the memory function has died sounds correct but I don't understand why the seat changes every time you use it or are multiple drivers changing it for their use?
In my case, once the memory was set and the last seat position was unchecked, I never had to readjust the seat.
Sorry I can't offer any insight into your problem.
2012 BMW 535i X-drive; Space Grey/Black Dakota, Fineline; Executive (Auto trunk, NAV, HiFi Prof, Lumbar, Rear Cam, SIRIUS, Voice), Technology (Blind Spot, Hi-Beam Assist, Lane Departure, Surround View), APPS, Rear heated seats, Tint.
Retired: 78 300D, 81 300SD, 87 190E, 88 Porsche 924, 89 260E, 93 300E, 97 Volvo 850, 98 E320, 03 BMW 325iT, 02 C230 Sport Coupe, 08 BMW 535i