It happened to me too, with the first BV9600 Pro that I bought.
They had to exchange it for another BV9600 Pro.
I received a call and the sound started loudly, but after 2 seconds it was low
Other times upon receiving the call, the ringtone constantly rose and fell
Also when listening to music in the Music application and listening to it through the rear speaker, the volume goes up and down all the time
The same in the YouTube application
I recommend you make use of the guarantee and have it changed.
This is another recognized bug
That's why we want updates !!!!
Thanks for your feedback, we are testing this now.
It's a shame because to process the warranty, I had to record a video, and I sent it to the seller, but since I was changing my phone several times, I can't find the video. If I find it, I'll upload it to youtube so you can see it