<cite>
I am using Process.Start(<MP3 file Name with path>) and able to run the
specific mp3 file but not be able to stop showing WMP. I want that I
will be able to play file but not want to display WMP interface.
</cite>

Hmm. Since you are really Windows Media Player as a separate
application it is hard to prevent the Windows Media Player interface
being shown. I guess you could:

1.    Start the Windows Media Player application as before
2.    Wait for Input Idle (not sure about the managed equivalent)
3.    Then send a VK_BACK to the Windows Media Player window (this will
involve doing a FindWindow() for a Window name/class which matches that
of Windows Media Player). This should cause Windows Media Player to
return focus to your application.
4.    Then when you want to stop playback, Kill the Windows Media Player
process. You won't really know when the song has finished playing
because there is no feedback from Windows Media Player.

<cite>
I am using Win32Window.SetWindowPos() method for setting the windows
position but it vein.
</cite>

Expected.

<cite>
Also the main part which I want to include in application is that I
will be able to find all mp3 files on smartphone and display in a list
so that I would be able to select and play it. Currently I am able to
find .wav files and display in a list and user is able to play selected
file(s).
</cite>

Windows Media Player 10 Mobile has a "media library"; not sure
about how easy it is to access this. Which version of the smartphone
are you targeting?

I think that it will fairly hard to make a general solution using
Windows Media Player to play your MP3 files; especially since a lot of
devices will be running Windows Media Player 9 (Smartphone 2003 pre
AKU2 and older). You may be best of porting ffMpeg or a similar
library. Alternatively you could try porting mp3sharp.

Vishal.