The decoder is working properly, that has nothing to do with the hardware limitation of the built-in sound card which is limited to 16-bit, 48KHz playback. So your files are being played back fine, they are just (losslessly) being output at 16-bit, 48KHz. Now, before you get upset, you should know that the process is lossless and unless you've passed a series of blind ABX tests, you aren't going to hear an audible difference.
You can bypass the integrated sound card and use a USB sound card/amp but that would be doing nothing more than fueling placebo at this point. A good USB amp costs around $150-$300 depending on what output you're looking for, they will cost more if you want placebophile (or audiophool, take your pick) standards of 24-bit, 192KHz.