OK, no need for python, and same as above except use mplayer instead of mpg321.

That's a line of text you can enter in the shell; to save it in a file as a shell script, prepend a shebang line like this:
The shebang tells the OS what to execute this script with; /bin/sh is the shell. You'll need to make the script executable withThen you'll be able to run it with or you can add it to e.g. Personal Menu, or make a .desktop file to get it in the Apps menu.

(And as for sleep mode, the tablet doesn't really have a sleep mode; when the processor has nothing to do, it stops, and once the screen backlight times out, it draws almost no power. An N800 in this state (fully powered on, and left to sit) will last about a month, and will wake up instantly on touching the the screen. This is the recommended state to leave the tablet in, as booting actually consumes significant power, several days' worth at idle.)