After plenty of headaches with audio sources it seems that this solution is best option:

It’s possible to play multiple sounds at once with ONE Audiosource. You can play up to 10-12 audio sources at once (only) by using PlayOneShot(); With it, Unity mix the audio output from the audio clip into a single channel. (Which is why it’s limited to 10-12 clip at once.

The key here is ONLY USING ONESHOT(). The problem comes when using Play() and PlayOneShot() form the same AudioSource.

So the player in the glitch machine has 2 audio source – one for movement and one for SFX that can play simultaneously. The AudioSources are assigned in the Inspector

    public AudioSource audioSourceMovement;
    public AudioSource audioSourceFX;

       /**********************************************
         MOVEMENT AUDIO (Audio Source 01)
        **********************************************/
        if (!audioSourceMovement.isPlaying)
        {
            audioSourceMovement.volume = 0.05f;
            audioSourceMovement.pitch = (float)(1.0f +
                (playerDistanceUp * 0.01) + (playerSpeed * 0.01f));
            audioSourceMovement.clip = moveAudio;
            audioSourceMovement.Play();
        }

	/***********************************
         FX AUDIO (Audio Source 02)
        ***********************************/
        if (!audioSourceFX.isPlaying)
        {
            audioSourceFX.pitch = 1f;
            audioSourceFX.volume = 0.2f;

            if (isBoosting)
            {
                audioSourceFX.PlayOneShot(boostAudio);
            }

            if (playerMovement.current_Power <= 0)
            {
                audioSourceFX.PlayOneShot(outOfPower);
            }
          
           //etc......etc.........