Did you compare the DVD with the CD using just the two-channel versions? If the DVD track was originally done in 5.1 and mixed down to two-channels by your DVD player or receiver, then the two-channel mixdown audio is going to have very different balances between the different sound elements, especially for those sound elements that got steered into the surround channels. During the two-channel mixdown from 5.1, those elements that bridge both the main and surround will overlap with one another and potentially drown out those sounds more anchored to the front soundstage.

And even with a dedicated two-channel mix, the producers might have chosen to remix that track as well, which would make the CD and the DVD sound somewhat different as well.

I'm not sure what you're talking about when you refer to the "siren effect". If you hear this only at very high levels, then it might be something in your room hitting a resonant frequency, especially if this phenomenon only occurs in your room and not elsewhere, like in your car or at your computer. Room treatments might not help in this case without knowing the origin.