Develop Reference

Is it possible in android to play an audio file over a phone call and receiver should hear it?

I search about it a lot but got nothing. I just want to know is it possible or not? If yes than how?

No.
The simple fact is the microphone / input audio stream during a phone call is not able to be pre-processed or replaced with another stream of any form.
So simply while one would think its easy to use media during a call, it simply is not possible due to the way the android system processes the audio.

How to bring audio from microphone to speaker on Android?

My problem is when I talk into the microphone, I want my sound to go to speaker of the phone which has Android OS. I think "MediaPlayer" class can't do it. Do you have any way?

Here you are not clear in which scenario you are going to do this but however it is possible to do using AudioManager, you can set your modes and audio services. For more details refer this link :
AudioManager

Streaming a call voice on android?

After a bit of googling I came to know that recording a call in android is not possible maybe because of some legal reasons, But I want to know whether its's possible to stream the call voice from android, I dont't want to record it, just want to tether it for some functionality requirement..
Thanks in advance...

accessing mic and earphone of android cellphone

hey can anybody please tel me that can i access a mic and earphone of my android cell,as we can access camera hardware of cellphone so is there any way to access them as i want to make a walky-talky app.And im a very beginner so please help me out thanks evry1.

You can use AudioRecord to record the audio.
Check some Android audio chat (SIP) projects to how they have done it:
http://code.google.com/p/csipsimple/
http://code.google.com/p/sipdroid/

Well that would be android.media.AudioRecord for the mic and android.media.AudioTrack for the sound. But what you want to do won't be something for beginner. You should at least be able to find this in the android api.
I suggest you to start with a less complex program. On the android developer site, you will find a good start for this.
http://developer.android.com/guide/index.html

How to make android phone ring?

I would like to make a android phone ring through code.
For example a app installed and the use recieves a call a app listens for this. If a certain number calls then a specific ringtone plays. How do i go about make code to make a android phone ring?

Well first of all i thought it was a good question but apparently people dont understand what you are asking. I think that you want to like call the phone you are calling from or somthing like that, i dont know it is possible but heres the tutorial on phone calls, from the official android website:
http://developer.android.com/guide/topics/network/sip.html

A ring is simply a sound file played through the speaker. Read up on how to configure your mediaplayer object to play your required sound through the external speaker.

This is part of the functionality of android anyway! I am positive that this is not possible to do with an android app without modifying the os. Apps cannot initiate incoming calls for security reasons.