I found that the sound volume is a lot lower than 5z, I have already turned on outdoor mode but still very hard to hear people talk when I am taking subway. Any clue?
I wouldn't say a lot lower. I just compared both models, but yes, the receiver on 5z is better. The receiver on ZenFone 7 is similar to the one used in ZenFone 6.
Zenfone 7's speaker is housed inside the flip camera module, the lack of space probably means a smaller earpiece speaker and as you mention 5z in your post it has a notch to house the earpiece speaker comfortably and that's the reason it's louder in my opinion
It's probably a limitation with the flip camera setup
You can refer to this post by the way