Raghu11

@RX400h, I haven’t tried yet, but will do. 
can we say the Amp is fine since it plays volume at least the first time after reconnecting battery? 

I doubt if this is related to Sony. I can connect the original stereo back and and I think most likely the result is the same. 

Regd working once after reconnecting the battery: It happens if the leave the battery disconnected for couple of minutes before reconnecting.

I checked the 'System Check' it shows 'NCON'. This happens even when the volume is on (first after re connecting battery).
The picture below shows AUDIO also 'NCON', I think that is because of aftermarket stereo.

I bought a used 2008 400h recently. It has Mark Levinson audio. I replaced the factory stereo with Sony XAV100 head unit with CarPlay. Everything was working fine.

Last week the battery died. I jump started couple of times and then replaced the battery. The problem is that there is no volume. Tried radio, bluetooth, CarPlay, usb etc. 
The strange thing is, if I disconnect the battery and then connect it, it works fine until next time. But when I stop the car and start again next time, there is no volume.

What could be wrong?

I followed excellent advice in post by @Mamoon here : https://us.lexusownersclub.com/forums/topic/66814-no-sound/ 
I have checked the fuses and they are ok.
The fact that it works every time after reconnecting battery might indicate connections are ok.