when I apply volume envelope to turn music audio volume down suddenly it becomes like test tone of high frequency and on volume meter is shown like 350+ db. what is going on?

I've never seen an audio envelope do anything like that. Can you give a step by step on how we could try to reproduce that?

I've got a very vague recollection of this being reported on the Sony Vegas forum (which is unfortunately down right now) some time ago.
Something to do with a zero crossing point as I recall.

I place music audio file below track with video footage and when I add point in volume line to lower music down it goes to beep of extremal volume AS SOON AS is pases that added point on volume line ( see audio Master reading).

the Sony Vegas forum (which is unfortunately down right now)

The forum just opened for me...

Loud POP sound on 2 envelope points at 90 degrees (http://www.sonycreativesoftware.com/forums/ShowMessage.asp?ForumID=4&MessageID=667156) is the relevant thread on the Sony Vegas forum.

Sony's official response:
You are experiencing this 'popping' noise because the audio envelope is being used like a cut, and cutting on a non-zero point will create a 'pop' noise in the audio. We have been able to reproduce this in all versions of Vegas, as well as in Sound Forge. Basically, it is a characteristic of audio editing. If you make a cut or drastically drop volume on a non-zero crossing point, you will hear a resulting 'pop'.

WHAT!
I am not "cutting" anything I am lowering SLIGHTLY volume of background music to hear narration; Interesting - when I reopen that project and reload that music audio file and do nothing, audio will become beep in very same spot like in last project. Is this like "memory effect"?
I had to move the music audio file to different audio track to play it normal.

Marius, Sony didn't say cutting, they said ...like a cut... which is a world of difference.

This was reported in Pro 9.0a and hasn't been a issue since.
What version are you using?
Have you tried moving the second point a tiny bit to the left or right to see if this cures it?

What are the media properties of the audio file that is having the problems?
From the attached screenshot, what you have layed out should behave normally.

It is regular mp3 file;