In addition to blitz, it seems to have started happening recently (last week?) for regular correspondence games as well.
I don't see an option to turn it off, though it doesn't really bother me.
At first I was looking all over, trying to figure out where the sound was coming from! (Phone? Computer? And in either case, which app?, etc., etc.)
If you go to "my settings" after clicking the upper right icon, there are sound settings located there but I'm not sure at the moment if this is where what you are hearing is coming from. (I just switched mine on but it will probably be a while before I hear anything as I don't have many games in progress)
Anyway, give that a look and maybe it will be the answer.