Hard time with pushing into arrays and foreach loops

getting closer to the solution of my problem... but now I am stuck again.

I am trying return all audio attachments linked to a post. Each post has an .ogg and a .mp3 file. Each file is an array. So separate arrays for the .ogg and .mp3 files. So I return their 'post_parent' value and use this to link them. I use this 'post_parent' as a key in my array. It works, but not like I want it to.

This is what I currently get, it is actually correct...

[18653] => Array (
    [0] => Array (
        [mp3] => 1.mp3
        [ogg] => 1.ogg
    )
)
[18612] => Array (
    [0] => Array (
            [ogg] => 2.ogg
            [mp3] => 2.mp3
    )
    [1] => Array (
            [ogg] => 3.ogg
            [mp3] => 3.mp3
    )
)
[18605] => Array (
    [0] => Array (
            [ogg] => 4.ogg
            [mp3] => 4.mp3
    )
)
[18592] => Array (
    [0] => Array (
            [ogg] => 5.ogg
            [mp3] => 5.mp3
    )
    [1] => Array (
            [ogg] => 6.ogg
            [mp3] => 7.mp3
    )
)

... but my code (at the bottom) can't do this:

[18612] => Array (
    [0] => Array (
            [ogg] => 12.ogg
            [mp3] => 12.mp3
    )
    [1] => Array (
            [ogg] => 13.ogg
            [mp3] => 13.mp3
    )
    [2] => Array (
            [ogg] => 14.ogg
            [mp3] => 14.mp3
    )
    [3] => Array (
            [ogg] => 15.ogg
            [mp3] => 15.mp3
    )
)

This is the code I use. It's weird, I know. I'm learning.

My problem is, as you can see, I use the numbers 0 and 1, I just hard coded them in. But it won't work if there are let's say, 4 files in a post (like my sample above). The files after the second found file will replace the $playlist_all[$parent_id][1]. So the [2] and [3] will not be created. And I exactly know why by the way, but I don't know how to make this stuff check if there is an key inside the array, and if so, increment it. I know about for loops and increment stuff... just not sure how it would work with this...

Again, I know where I am going wrong, but this is the only solution I can think of. I just check if there is a [0], if not create it. If there is a [0] I create the [1].

foreach ( $attachments as $attachment ) {

  $parent_id = $attachment->post_parent;
  $title = $attachment->post_title;
  $type  = $attachment->post_mime_type;

  if ($type == 'audio/ogg') {
    if (!$playlist_all[$parent_id][0]['ogg']) {
      $playlist_all[$parent_id][0]['ogg'] = $attachment->guid;
    } else {
      $playlist_all[$parent_id][1]['ogg'] = $attachment->guid;
    }
  } elseif ($type == 'audio/mpeg') {
    if (!$playlist_all[$parent_id][0]['mp3']) {
        $playlist_all[$parent_id][0]['mp3'] = $attachment->guid;
    } else {
        $playlist_all[$parent_id][1]['mp3'] = $attachment->guid;
    }
  }
}

I hope it's clear, it's messy code, but I got it to work. Well for 50%.

Thanks in advance.