Triple your audio volume in MPlayer
Some videos you download might not have such a high audio volume, even with alsamixer set to 100%. There’s a trick for this. Start your video with
mplayer -softvol -softvol-max 300 video_file.avi
to boost your volume 300%. Substitute 300 with any number ranging from 10 to 10000. Be careful not to break your speakers.