Jitter does not reduce the number of bits. There are many kinds of jitter. Jitter in the channel signal is accounted for in the readout circuitry. Jitter in the digital audio output is related to the audio circuitry and is independent of the DVD format. It's true that jitter at different stages can reduce audio quality, but comparing it to a 12-bit sample size is naive and inaccurate.
Numerous reviews indicate that most DVD players sound as good or better than most CD transports.
Audio level too low. In truth the audio level is too high on everything else. Movie soundtracks are extremely dynamic, ranging from near silence to intense explosions. This is why the line volume from DVD players is lower than from almost all other sources. 133 minutes per side.