No voice audio on smartphone with one speaker

Solution:
For the audio, I had to choose mono instead of mid-side in the Stereo Fixer. Now, the voice is very clear and audible on the smartphone.

I do not understand the reason for this.


System specs - Windows 10
Davinci Resolve Studio 18.1.4
Footage specs - MP4, 4K UHD

Hi all,
I created video for Youtube in Davinci Resolve. I did the loudness adjustment so Youtube does not perform any normalization.

The video has some background wave sounds and a woman speaking the subtitles. When I play the video on my PC, iPad, or Smartphone with headphones, everything is fine. But when I play the video on my Huawei P30 Pro (one-speaker-phone), the voice is not audible.
I am worried that the audio will not be audible for many smartphones with one speaker. I do not have this issue with any of my other videos that have a voice included.

I also tried listening to only the right or left channel of the headphones connected to the smartphone but still could here the voice. I also tried exporting the video first as DNxHR with one audio line and then to MP4, but still the same issue. I also tried to export the spoken audio first as MP4 audio, add it back in as separate audio line in the project and export this, but still no luck. I also checked in VLC Player the audio and it is stereo with 2 channels, 48KHz, 32Bit.

I recorded the voice with the Rode Wireless Go 2. I have the feeling that there was something going on with the 24Bit vs ??Bit. My second guess would be that Youtube provides for one-speaker smartphones a separate audio which is heavily processed and cut in dynamic range? In my tests, I can barely hear the voice as it sounds like being put through heavy noise cancelling. It might be also related to the loudness of the voice.

Somehow this issue must be related to Davinci Resolve since I do not have any issues with my other videos.

Here is the private video link to the YouTube video. I also attached the video to this post.

Solved:
For the audio, I had to choose mono instead of mid-side in the Stereo Fixer. Now, the voice is very clear and audible on the smartphone.

I do not understand the reason for this.

Could be a phase issue.

Could be a phase issue.

My guess is that the smartphone is recording stereo audio, and if it gets summed to mono, it's just out of phase enough to cause level problems. For a lot of reasons, we tell people "don't use smartphone audio if possible."

The Rode, being a mono source, does not provide an M/S signal. The M/S setting in Stereo Fixer is for varying the width of Mid Side audio which is recorded to two channels, Mid on the left and Side on the right. Decoding M/S adds the Side signal to the Mid both as is and 180 degrees out of phase.
It is normal to record the Side signal using a microphone with a figure of eight polar pattern which picks up both in and out of phase audio.
24bit will not affect the audio other than giving it more bandwidth.