I recently started using Windows Media Player 9 to decode a few different HDCD discs that I own.

What I noticed immediately was how when decoded, the same exact songs that I ripped to .wav are about 4 or 5 dB less loud??

I kept switching back from the non-decoded .wav file to the decoded HDCD and it was very evident with all of my discs and every song.

For some reason I thought I read on the HDCD website that decoded discs would actually be up to 6db louder because of the extra 4-bits??

What's going on?

Could be due to the fact that my computer soundcard is only 16-bit?

A HDCD decoded with a player supporting it (WMP does) will use the extra information to extend dynamic range. Since normal CD's tend to be mastered to be as loud as possible, without any regard to sound quality, it's logical the HDCD playback will be quieter - the extra dynamic range must be used somehow, and the CD will already be as loud as possible on your system.