Hi.
How to change the gain in a ALLEGRO_VIDEO?

Audio gain? You can adjust the gain of the mixer the video is attached to:

al_start_video(video, al_get_default_mixer());
al_set_mixer_gain(al_get_default_mixer(), 0.5);

That will affect all sounds playing through that mixer, so you could create another mixer and attach the video to that mixer instead.

Thanks for Reply!!