Videos play without sound on Android device

I have an AIR app playing embedded videos on an Android mobile device. The app works fine, and the videos play, but no sound can be heard.

It used to work previously, and so I don't know why it won't work any more. Any idea?

Here are the details:

– AIR app compiled with latest SDK (32.0.0.116).

Tested with older SDKs (31, 30), but it doesn't fix the issue.

– I use the Video class. I've tested StageVideo instead, but it's reported as unavailable on the device.

I've also made a dedicated mini-app to get rid of all other possible issues from the actual app I'm working on.

– I've tested various combinations of manifest additions, such as :

<containsVideo> true or false

<disableMediaCodec> true or false

android:hardwareAccelerated true or false

– The device is a Samsung Galaxy Tab A tablet with Android 8.1.0. When played outside the AIR app on the same device, the videos are played correctly, WITH sound.

I've also tested on a Nexus with Android 7.1.1: same issue.

– The videos are mp4 files. I've tested several audio encodings (from Adobe Media Encoder):

AAC, AAC+ Version1, AAC+ Version2, MPEG-1 Layer I, MPEG-1 Layer II.

The mp4 files are packaged with the apk file (included inside the FlashBuilder project). The mp4 files' sizes are between 10-20MB.

mp4 sample files from the web do not work either.

– If I convert the file to FLV, with soundtrack encoded in mp3 (not AAC), it immediately works fine.

I know that this file format is obsolete (I had to use an online converter, since Adobe Media Encoder doesn't provide the FLV option anymore), but unfortunately, it looks like my only solution at this point...

Any help or suggestion would be appreciated. Thanks!