Canonical way to use mplex with lpcm sound seems to involve sox

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1. sox NAME.wav -t raw -x -s -w -c2 -r48000 NAME.lpcm
2. mplex -S 0 -f 8 -V -o NAME.mpg NAME.lpcm NAME.m2v

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I also found at mjpeg-users list this utility that makes wavs into lpcm

only 48/96 khz, 16/24bits stereo wavs are supported

Also, this little table suggest there IS dynamic range bit in lpcm header

dynamic_range X 3 X High 3 bits of byte.

dynamic_range Y 5 Y Lower 5 bits of byte

linear gain = 2^(4-(X+(Y/30)))

in dB gain = 24.082 - 6.0206 X - 0.2007 Y

Also, according to this post 24-bit lpcm worked around mjpegtools-1.8.0 release:

all this quite theoretical at this point, not tried it yet.