If you change the frame rate you change the audio playback. No way around that.
The easiest way to ramp to slow mo is to use Time Remapping. Enabling time remapping will set a keyframe at the beginning and ends of your comp. Make sure that your comp is long enough to accommodate the slow mo. Set a keyframe where you want the speed change to start and then drag the last keyframe down the timeline. Adjust the ramp using the graph editor. It's fairly straight forward.
If you want to start at normal speed, ramp to slow mo, then return to normal speed you have to set a keyframe for the start and the end of the slow mo, then when you drag out the slow mo section you have to select the last two keyframes.
Be aware that changing the speed of a clip and editing the time remapping in the graph editor requires you to think differently. It takes a while to get used to the way things happen in the timeline when you start adjusting the frame rate of the playback. The graph is showing you the real time in seconds as the clip was recorded. The flatter the line the slower the video will play back. The steeper the line the faster.
This example shows a clip that plays back normal then ramps to a slow section, then ramps back up to normal speed. Without 3rd party software this is your easiest solution.
Your use of terminology is odd, to say the least, so it's hard to even understand what you actually mean. Explain better and clarify. I can at least answer one point: Yes, you will see shifts in audio pitch. AE simply is not an audio processing tool.
Sorry for the confusions caused,
What i want to understand:
What will happen to audio and video, if we follow some steps as in the below youtube link?
The time strectch is happening both for audio and video.
Are there any other impact apart from time stretching like pitch shifting or scaling?
Increasing the time length will increase the lenth of the video file. Am i right?
Why don't you try it? The tutorial kind of gave you an idea of what to do but the technique was a little sloppy. I think this is a better way than editing the speed graph:
Also, if you play back slower it will take more time. Just play with time remapping using any footage. I just grabbed a quick bit of low rez cell phone footagen from a cheapie phone for this example shot from my bike on the way home tonight.