sound goes loud and then quite then loud quite ect

[celticfc]

hope im posting in right section

I have zgemma 2s, i have been trying some test lines ( not got 1 yes )over past week and my prob is that sometimes ...
when i watch some channels the sound goes loud and then quite then loud quite ect. , is this a line fault or a box configuration fault
I dont want to get test line if its not right
thanks

ps running openvix ver. 4.2