var song = 0;
if(key_f1 == 1)
{
    song += 1;
}

function song_startup()
{
    while(1)
    {
        if(song == 1)
        {
            media_play(REPLACE_THIS_WITH_YOUR_OWN_STUFF);
        }
        if(song == 2)
        {
            media_play(REPLACE_THIS_WITH_YOUR_OWN_STUFF);
        }
        if(song == 3)
        {
            media_play(REPLACE_THIS_WITH_YOUR_OWN_STUFF);
        }
    }
}