I want to play video (3gp) without sound, leaving the sound channels for playing another audio.

According to my test, neither the video and the audio has sound when both of them are played at the same time.

I have also test the following scenarios with no luck:
1. set video player to mute.
2. play an audio disabled video clip.
3. set audio to leftmost channel and video to the rightmost. (The audio plays well while the video will not play anymore)

I welcome any thoughts/discussion on this issue.