A video of the test rom in action was posted at nesdev, linked above.

http://gbdev.gg8.se/wiki/articles/CGB_R … ad_Only.29
"These are undocumented CGB Registers. The numbers in brackets () indicate the initial values. Purpose of these registers is unknown (if any). Registers FF6C and FF74 are always FFh if the CGB is in Non CGB Mode. Each of the four nibbles in registers FF76-FF77 represent the current PCM amplitude (00h-0Fh) in each of the four sound channels, where the lower nibble of FF76 represents the channel 1 and the higher nibble in FF77 represents channel 4.
"

Some channels are for amplitude. I wonder what the other registers are for.

CGB only, I believe. Sample in general means to sample a signal at the current point in time. Sampled sound means that you do this periodically and save the result. What this does is to sample the state of one of the channels at one point in time.

You're saying that these could be researched to make audio emulation more accurate?
What does this mean:
"allow reading the current PCM samples of each of the 4 APU channels in real time"

These two pulse waves and noise are not samples, are they? Noise is created with a feedback register, or so I thought.
Are the registers present in all CPU revisions?
These registers are read-only?

Yes! I have a use for these in a previously mentioned and not completed project. They could be used for making test ROMs to improve emulators by making their emulation audio phase correct.

http://forums.nesdev.com/viewtopic.php?f=20&t=13836

Just came across this. This is pretty big news imo. Could these registers be used in applications other than visual?