Capacitors and filtering
1) the reason there is a 16KHz cut off: 
While our ears do have a hearing range of 20-22KHz the actual range that we use in our every day life for hearing is about 6-10KHz (at most) 

If you are listening to music, one of the things to realize is that pitches, musical notes use a logarithmic scale. so A4 is at 440Hz while A5 880 Hz A6 is at 1760 Hz what this means that there are only a few notes are left to be able to play above 16kHz. Just for reference, the highest note played on a piano would be 7.9kHz which is a B

http://peabody.sapp.org/class/st2/lab/notehz/

2) 100K ohm resistors are cheap and common, same-thing with the capacitor. its the most cost effective choice.