You can attach several AudioSources to the same object in the Inspector, and get them in an array at Awake with GetComponents(AudioSource) - but knowing who's who may become a problem, since there's no component identifier.
A better alternative is to add the AudioSources at Awake - the script SoundController.js in the Car Tutorial adds as many as 10 AudioSource components to the car!
You could do something like this:

// define the audio clips:
var clipAmb: AudioClip;
var clipEngine: AudioClip;
var clipWeapon1: AudioClip;
var clipWeapon2: AudioClip;

private var audioAmb: AudioSource;
private var audioEngine: AudioSource;
private var audioWeapon1: AudioSource;
private var audioWeapon2: AudioSource;

function AddAudio(clip:AudioClip, loop: boolean, playAwake: boolean, vol: float): AudioSource {
  var newAudio = gameObject.AddComponent(AudioSource);
  newAudio.clip = clip;
  newAudio.loop = loop;
  newAudio.playOnAwake = playAwake;
  newAudio.volume = vol;
  return newAudio;
}

function Awake(){
  // add the necessary AudioSources:
  audioAmb = AddAudio(clipAmb, true, true, 0.2);
  audioEngine = AddAudio(clipEngine, true, true, 0.4);
  audioWeapon1 = AddAudio(clipWeapon1, false, false, 0.8);
  audioWeapon2 = AddAudio(clipWeapon2, false, false, 0.8);
}

You can then use the audio sources like this:

  audioWeapon1.Play();
  audioEngine.pitch *= curVelocity/maxVelocity;