CBlue: On KGS, teaching games may support one-way voice transmission. That means the teacher (exclusively) may use a microphone and everyone observing the game will hear him speaking.
However, more often than not the voice quality is very poor, making it hard to follow the lesson, especially for non-native speakers of the teacher's language.
I decided to make a list of problems and solutions, related to voice lessons on KGS, in the hope of teachers actually reading this or possibly being pointed to this in order to fix sound quality for their games:
Problem (most common): Audio is too quiet.
Requires turning up volume so far that stone clacking sounds like thunderstorms and any other sound effects make you fall off the chair.
Reason: Either the teacher is too far away from the microphone, or most likely on Windows OS the microphone volume is too low and "microphone boost" is not enabled (it's disabled by default).
Solution: For Microsoft Windows: Double-click the yellow speaker symbol in the task bar, and the mixer window should pop up. In the menu 'Options' choose 'Properties'. Another dialogue pops up. Choose 'Recording'. Now you should see a couple of tabs next to each other, representing all recording devices. One of it says "Microphone". Below its slider there should be a button ("Extended" or similar), click it! Now you are able to activate "Microphone boost", which should increase the volume significantly.
For OSX: RobFerguson It can be difficult. You can overdrive the audio in software, but it will require a 3rd party sound application like AirFoil?, or Soundflower with some sort of mixing software.
Problem: Audio is distorted.
Reason:The microphone is too close to the teacher, possibly also intercepting breathing. Or the microphone is of poor quality, true for cheap devices.
Solution: Buy a better microphone. Try to position the microphone beside your face instead in front of it and see if it improves the quality.
RobFerguson: The major problem is that the teachers do not know how to set audio levels. There are many tutorials on this subject under recording podcasts. Here is an example: http://www.jakeludington.com/podcasting/20050222_recording_a_podcast.html . Considering the nature of the problem (teachers don't know how to use microphones and may have trouble learning), the software could be improved to make setting levels easier. At the very least an overdrive feature like audiogolessons.com has would be useful.
 Teacher's poor English pronunciation is another difficulty.