When listening to audio on a wireless device such as AirPods, you may find that on occasions the volume level of audio playback will drop when your device goes to sleep.
When this occurs, the volume level indicator on your device does not change, just the level of audio playback itself.
Note that this is specifically the volume of the audio that you are playing, not the volume level of VoiceOver.
This behaviour does not appear to be related to a specific app or type of audio. It has been experienced whilst listening to podcasts in different podcast apps, whilst playing books in Voice Dream Reader, and when listening to streaming radio in apps such as Broadcast and TuneIn.
This behaviour does also not appear to be specific to certain headphones - it's been consistently experienced on 1st and 2nd Generation AirPods, AirPods Pro, and AfterShokz headphones.
The behaviour has not, however, been experienced when listening to audio through the device's speaker.
On occasions the volume level will spontaneously adjust back to the correct volume level - some times after a couple of minutes, and at other times much longer, such as 30 minutes after the device went to sleep.
What will immediately correct the volume level, is turning VoiceOver off. This would appear to strongly indicate that this is a VoiceOver-related issue.
Whether VoiceOver's Audio Ducking feature is enabled or not does not make a difference (not that it should cause this behaviour, but it does suggest that this feature is not some how involved).