OP listen up, open ~/.mplayer/config with whatever text editor you use and put this in there (if you don't have an empty file named config, make one).
#alang=en,eng >uncomment if you want english dub
slang=en,eng >this starts english subs automaticly
monitoraspect=1280/720 >prevents ASPECT RATIO, replace with your own resolution.
lavdopts=fast=1:skiploopfilter=all >based on what you said earlier your computer is slow as hell, this will let you run .mkv files without any slowdown, sound will be perfectly synced