I'm no plumber but gasses shouldn't build up in the wall unless there is a break in the soil pipe.
I am a roofer and I have fixed this sewer smell a few times by removing birds nests from the top of the soil pipe, which was clogging the pipe.
I am a roofer and I have fixed this sewer smell a few times by removing birds nests from the top of the soil pipe, which was clogging the pipe.