In The Walking Dead TV show, I noticed that every time there is a cricket noise that gets louder. Walkers are almost always nearby. Can somebody confirm or deny this? Is it related to anything?
The Walking Dead is shot in Georgia, in different towns according to the season. That cricket noise you are hearing is being generated by cicada. It's almost impossible to scrub from outdoor shots due to the large amount of frequencies the sound covers. As the first three seasons were shot in the summer when the cicada are at their peak. Here's a video that can show you how loud they they are and how sustained it is.
The reason that it seems to get more prevalent is the lack of back ground noise for it to contrast with. The Walking Dead often has the music and other sound effects go quiet for the scare value. That means that the normal background noise surges to the forefront.
The reason for the wave effect of the ambient sound is to do with the network processing the sound before broadcast, probably to compensate for the ridiculous sound found in most commercials. It will be a combination of compression and expansion on the original audio track. I would wager the DVD sound will be substantially better, or even if viewed on a different network.