This question has nothing to do with sound waves, physical strings or walls...
I am reading: https://arxiv.org/abs/1709.07091
"In the post-inflationary PQ symmetry breaking scenario, axions can be produced from the decay of string-wall systems, and we must take account of such contributions quantitatively."
I am not sure what they mean by decay of a string wall system.
Can someone please explain this?