motley: Why is the volume on my Sony BDPS550 so much quieter with a blueray compared with a DVD?
It isn't, it's more dynamic. What that means is HD audio has a larger range between the quiet bits and the loud bits, compared to old style Dolby Digital or DTS. In theory this makes the sound "better" but in practice it does seem to mean people spend a lot of time reaching for the volume control.
You should have some options for compressing the dynamic range to bring the quiet bits and the loud bits closer together, volume-wise. Whether you'd want to do this is another matter.