I recorded video file as MP4 from my mobile phone,the video is ok but the audio quality is poor.I have audio file recorded seperately through PA system.Now i wan't to remove audio from MP4 file and add new mp3 audio recorded through PA system. Please let me know how to do this and is there any free program available for this purpose?Please help.

Thanks,
Sunny

you can probably import the video into a video editor, mute that clips sound, and then just import the sound clip and make sure to match the timing up. Windows move Maker might even be able to do it (which ships with all windows xp and assumedly something equivalent on vista).

When you say MP4, do you mean 3gpp? If you have problem opening the file using any editor such as Jeigh suggested windows movie maker, you can use SUPER to encode it to your desire format or the format that's support by the editor. This this case probably WMV.

http://www.erightsoft.net/S6Kg1.html

You can also try virtualdub, which is more powerful, and it's opensource. But it's a little steep on learning curve.