I was still struggling with erratic sound volumes even after making this change:
This is possibly Windows thinking that the ATC voices are some sort of telephony. A possible solution is to right-click on the speaker icon in the tray, click on Sounds, select the "Communications" tab and change "When Windows detects communications activity:" from "Reduce the volume ..." to "Do nothing".
I found the following 'fix' that seems to have sorted things out:
In Settings/System/Sound, select 'App volume and device preferences' at the bottom of the page under 'Advanced sound options'; at the bottom of the next page select 'Reset'.
Equalising the sound levels for all the various apps sorted out my remaining issues, and overall volume can still be controlled with the 'Master volume'.
Hope this helps,