The Xfinity box doesn't have volume control. And another sometimes issue is audio sync to video when using this choice.
And a interesting side note when watching the football game yesterday we had 2 different boxes, tablet via xfinty and TV using Chrome cast tv and chrome didn't have commercials and almost 40 second time difference across the choices with the DVR box being last.
And now you know how I won my $5 bet

So if RCA on TV check it's audio settings.
Alan