To recover the hidden sound in the Beethoven:
1) Obtain the list of data point, on a scale from 0 - 65535 (in Mathematica, data is read as signed real numbers, so add 0.5, then multiply by 65536)
2) Drop 12 most-significant bits (Mathematica: mod list by 16)
3) Return to proper format (Mathematica: divide by 16, subtract 0.5)
4) Play as sound (sampled at 44100 Hz)
To recover the hidden cat in the other cat:
1) Import the "twocats" file. It should give a list of 480x640 3-byte pixels (Import["â€¦twocats.bmp","Data"] to extract the data)
2) Display the mixed picture (Image[twocats,"Byte"])
3) Extract the lowest order 4 bits from each byte, then rescale (Image[16 Mod[twocats,16],"Byte"])