Taking a shot in the dark(Indian English to American English)
I think the problem(which happens here in the US as well) is when you change channels the source changes from digital to analog sound. And even from that difference there is digital 5.1 and digital 2.0. So, when you change channels there is a brief instance of the AVR having to figure out what it is being sent to it.
Same thing happens(if this applies to you, try it) if you play a CD in a BD player over the HDMI* connection. Every time a track ends till the next track that "dead air" resets the AVR and when the next track starts it takes a second or two to realize what is being sent.
*The way to resolve audio black outs for CD in a BD player is to use the second audio output of the BD(if your BD player has one) into the AVR. Then the AVR doesn't have as much to do figuring out what is going on.