If OSX is anything like other OS's, you have to turn down the volume through OSX, not mplayer. mplayer's volume control is a way to "fine tune" the system volume. It sounds like your system volume is set high to begin with. Lower it. Then launch mplayer.
I would tell you how but I don't use OSX. Surely someone else can tell you, or you could learn yourself. It's worth learning how to use mplayer, if you are serious about audio.
I would tell you how but I don't use OSX. Surely someone else can tell you, or you could learn yourself. It's worth learning how to use mplayer, if you are serious about audio.