I realize that this is a reply to an old post, but I spent quite some time trying to solve this issue as well and found no definitive answers on the web.
The closest thing I found to an answer is this post from 2018:
“On Android, the media audio output is always played through the OS sound mixer at the moment, so that attenuation is not possible there.”
I suspect that this is still the case (I am using Unreal Engine 5.4) as I had attempted to fake a bit of attenuation by using trigger volumes to fade in/out the audio and discovered that the Audio Fade In/Out and Adjust Volume nodes also have no effect on Media Sound components when running on the Quest. The only node that does seem to work is Media Player Set Native Volume (which does not have fade options).
I also extracted the audio from the movie files and played them with an Audio Component attached to the audio source actor (as synchronization is not an issue for me in this case) but did not remove the audio from the movie files and discovered two things:
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Even without any Media Sound components, the audio from the Media Player played in 2D at full volume causing me to believe that attenuation was not working (it was simply covering it up). My solution was to use Set Native Volume to mute the media player, at which point I discovered a second thing:
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Even with the Media Player muted in app using Set Native Volume, when recording a video of the VR application (on Quest 3s) the Media Player audio is recorded at full volume.
Note that all of the attenuation and volume controls work fine in editor on Windows.
Having any kind of documentation from Epic or Meta would been helpful.
I hope this post helps others with this issue.