Hi,
I'm Couin from France so sorry if my english is not easy to understand 
I'm built a mini PC for playing mp3s in my car.
I use MPXPLAY ( http://mpxplay.sourceforge.net/ ) that is a great DOS mp3 player.
MPXPLAY drives a LCD via LPT1 Port.
I'm blocked on the command point. I could hack a keyboard and map buttons to PCB but it's not very nice and it is limited by number of buttons.
MPXPlay should be able to be driven by Serial Port, so I would use an IR Remote to do it.
In the .ini file of mpx, we can set parameters and codes, so the interesting area looks like that :
[serialport]
SerialEnable =1 ; - set to 1 to enable serial control
HandlerCFG =UIR,COM1,9600,6 ; - configure a handler
SerialFunc =45ba00ff0000,0b30 ; '0' - set functions (buttons) for this handler
SerialFunc =45ba807f0000,0231 ; '1'
SerialFunc =45ba40bf0000,0332 ; '2' These values are configured
SerialFunc =45bac03f0000,0433 ; '3' to my remote controller.
SerialFunc =45ba20df0000,0534 ; '4'
SerialFunc =45baa05f0000,0635 ; '5' You can declare max 99 SerialFunc lines (buttons)
SerialFunc =45ba609f0000,0736 ; '6' for one handler.
SerialFunc =45bae01f0000,0837 ; '7'
SerialFunc =45ba10ef0000,0938 ; '8'
SerialFunc =45ba906f0000,0a39 ; '9'
...
Problem, how to know me remote codes ? I found a script that reveals dec and hex codes received, using IRremote.h library.
For example, "Power" key of my remote is seen as 367671495 (DEC) or 15EA38C7 (HEX) .
Even if I set theese code in a SerialFunc parameter, it does nothing.
I noticed that IUR param was for a module (because behind MPX, it says that it can't be initiailized) .
I can change for COMC, or VT100 instead of UIR.
I tried a lot and a lot of scripts for arduino sendings serial commands but nothing great
Just one script made song restarts from begining but it wes not what I asked. (But it confirms that COM1 is ok and mpx listen on pin2 ).
I spent so many hours to search and today I still have nothing better than when I started
I someone could help me about communication between Arduino and MPXPLAYER, it would be very great 
My arduino is mega 2560.
Thanks !!!
Couin