This is pretty much just a note to myself, hoping that I'll find it through the tags if I need it again ...
use File Source (Async.) → MPEG-2 Demultiplexer → Nero Audio Decoder 2 → WAV Dest → File Writer
The result is a Signed 24 bit PCM 48kHz WAV file that the FLAC 1.2.1 decoder can work with.