Hi,
I wanted to kindly ask for an explanation on how to start audio files on windows media player with Arduino. I need a sketch that randomly selects files from a folder and runs them with media player. I have read the previous threads on this topic but still I haven't understood how to make the two of them communicate.
Thanks in advance to anyone willing to help. 
What threads have you read?
Post links please.
First off.. you would probably need to use an Arduino that is capable of acting as a HID (human input device)..
*(like a mouse or keyboard..etc)
this will allow you step 1 of having your external device/circuit be able to communicate with your PC
The next bit/step is a bit tougher....
How do you start, control and set a file for WMP to use?
Is there an API? Some sort of shortcut you can do this on your PC (without an Arduino?)
Alternately.. a regular Arduino (UNO) sending data via serial port to the PC could also work..
however. you still need an interpreter app and a way to send these actions to the WM player..
unless you have a server and aware of the programming and network protocals behind it,
your simple line of communication between a windows pc and an arduino is a serial connection. if you plug an arduino board into a PC with a USB cable windows automatically recocnizes it as an available serial communication port.
you would be sending bytes back and forth and need to program both the arduino and the PC to intemperate the communication
you would then need to install visual studio to write your windows program that would talk to your arduino through the serial connection. if you write your windows program in c# then there is a library for talking to a serial port and also simple functions built in to make windows media player play a file (or make any program open any file). the sky is the limit once you have an executable windows PC program receiving instructions from an arduino.
once you get visual studio installed on your PC and firgure out how to make a new form app running let me know and i could help you with the coding.