Resample to 12bit PCM sound?

I have trouble finding a program wich will resample 16bit 44.1KHz PCM into 12bit 25Khz. There's not much info on this to be found...

Anyone have a clue? I tried audacity, ffmpeg,... but to no avail. I was thinking about reducing the amplitude on a 16bit sample by 75% in a normal editor and throw away the highest nibble but something tells me it might not be that simple...