You should use OpenGL. OpenGL does 3D graphics. Not less, not more. If you need to do other things besides 3D graphics, like for example accept input from the keyboard and mouse, play sounds and other things, you should also use Allegro. Allegro does keaboard, mouse, joystick, sound, music, timers and all those other things. To be able to use Allegro and OpenGL together you should use AllegroGL. AllegroGL makes it possible to use Allegro with OpenGL. To learn about OpenGL you should read the official documentation, called the blue book (programmer's reference) and the red book (programmer's guidlines). You should also read NeHe's OpenGL tutorials.

Go back and strike 3D graphics out of your schedule. Because if you want to learn how to play mp3, ogg, midi and other music files by yourself, then be prepared to spend a lot of time learning. At least a few years...

Emphasis mine.

If you are wanting to make a library that others use to play all kinds of file types, that's one thing, if you want to make some source that can handle all your music needs, that's another (because the second option is free to use available libraries and it's for you). For example, I made a dj.h and dj.cpp file that handles my music needs by interfacing with the alogg library. I just wanted one file to talk with the library so all I had to do for the rest of my program was talk to the dj. It was a pretty quick hack, so it doesn't handle a lot of things (like MIDI or mp3), but it get's the ogg files going and that's what I needed. I just add things to the dj's playlist, tell it if I want it set for shuffle and then let it do it's thing. Food for thought.

Something like this schould work.

if ( !keypressed() ) { pressDelay = 0; }
      

if ( keypressed() ) 
{ 
  if ( pressDelay == 0 ) {
    handle_input();
    pressDelay = 20;
  }
}
if (pressDelay != 0 ) pressDelay--;