It is not WAI.
I think this issue is caused by how media views are flattened.
I'll work on this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/35ec8d11347ddcdc425f63b8da279f3c3fcfaf92

commit 35ec8d11347ddcdc425f63b8da279f3c3fcfaf92
Author: Naoki Fukino <fukino@chromium.org>
Date: Wed Aug 08 06:21:35 2018

Files app: Retrieve all files under media-view's root to provide flatten views.

In FileSystem API level, the "Audio" media view has a directory structure like
Audio/{Artist}/{Song}.aac. We can't limit the level of recursion up to 1 to
find entries for "Audio".

Actually, a file will not be shown in multiple folders in media view hierarchy,
since no folders will be added in media documents provider.
We don't need to limit the level of recursion.

Bug:  870532 
Test: Manually tested on arc-enabled kevin with voice recording app installed.
Change-Id: I1087b9c6e11812025d8456eef8dbec9faeb29cd0
Reviewed-on: https://chromium-review.googlesource.com/1164865
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581484}
[modify] https://crrev.com/35ec8d11347ddcdc425f63b8da279f3c3fcfaf92/ui/file_manager/file_manager/foreground/js/directory_contents.js

This bug requires manual review: M69 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), kariahda@(iOS), cindyb@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Merge approved, M69.

Issue 865126 has been merged into this issue.

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

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For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e5ab0dd1f1a35ee151e584e1773e2fa0442bd399

commit e5ab0dd1f1a35ee151e584e1773e2fa0442bd399
Author: Naoki Fukino <fukino@chromium.org>
Date: Tue Aug 14 05:19:09 2018

Files app: Retrieve all files under media-view's root to provide flatten views.

In FileSystem API level, the "Audio" media view has a directory structure like
Audio/{Artist}/{Song}.aac. We can't limit the level of recursion up to 1 to
find entries for "Audio".

Actually, a file will not be shown in multiple folders in media view hierarchy,
since no folders will be added in media documents provider.
We don't need to limit the level of recursion.

Bug:  870532 
Test: Manually tested on arc-enabled kevin with voice recording app installed.
Change-Id: I1087b9c6e11812025d8456eef8dbec9faeb29cd0
Reviewed-on: https://chromium-review.googlesource.com/1164865
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#581484}(cherry picked from commit 35ec8d11347ddcdc425f63b8da279f3c3fcfaf92)
Reviewed-on: https://chromium-review.googlesource.com/1173872
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#608}
Cr-Branched-From: 271eaf50594eb818c9295dc78d364aea18c82ea8-refs/heads/master@{#576753}
[modify] https://crrev.com/e5ab0dd1f1a35ee151e584e1773e2fa0442bd399/ui/file_manager/file_manager/foreground/js/directory_contents.js

Able to reproduce the issue on M70 (11021.19.0, 70.0.3538.22 dev-channel eve).
Files recorded using voice recorder app are displayed in Play Files > Recorders folder but do not appear in 'Audio' folder of Media views section

Re-opening the bug.

fukino@, can you check as this is Pri-1?

Re: #11
I was not able to reproduce the issue using the same version (11021.19.0, 70.0.3538.22 dev-channel eve)

sdantuluri@, I have some questions.
- Do you see any error log in inspector? You can open the inspector by Ctrl+Shift+J on Files app and choose "Console" tab.
- Have you tried other apps than "Voice Recorder" by quality apps? No apps worked?
- Do "Images" and "Videos" in media view section has any contents?

fukino@

- Do you see any error log in inspector? You can open the inspector by Ctrl+Shift+J on Files app and choose "Console" tab.
  
No error log was observed in inspector.

- Have you tried other apps than "Voice Recorder" by quality apps? No apps worked?

I used some voice recorder apps listed below
1. Easy voice reocrder (https://play.google.com/store/apps/details?id=com.coffeebeanventures.easyvoicerecorder). Audio .wav file saved in Play files -> EasyVoicerecorder folder
2. Smart recorder (https://play.google.com/store/apps/details?id=com.andrwq.recorder). Audio .wav file saved in Play files -> SmartVoiceRecorder folder
3. Voice recorder by Splend Apps (https://play.google.com/store/apps/details?id=com.splendapps.vox). Audio .m4a file saved in Play files -> VoiceRecorder folder

The .m4a audio files appeared in the Audio tab as .mp3 files and able to play them.
The .wav audio files do not appear in Audio tab.

- Do "Images" and "Videos" in media view section has any contents?
Yes. Contents are displayed.

Thank you for the answers.

The issue you see seems to be WAI.
Recorded .wav files by Easy voice recorder and Smart recorder are not displayed in Android file picker's Audio tab.
This means, these apps do not register their recorded files as Audio files in Android system.

Files app shows the same set of Audio files as Android file picker does, so it is intended behavior.