As I recall, the sound wasn't bad before I changed the calibre from .177 to .25. However, when changing the calibre, I decided to dismantle the gun as well to see the internals, and when I reassembled it, the gun won't cock. I got some help from this forum about that issue but I still struggle with the problem. So I have no idea how loud it is in .25.
I don't know how the shroud can muffle the sound produced by an airgun, either. But I found this shroud damper on STO's website
https://www.silentthunderordnance.com/air/shroud-damper, and I'm posting this question just in case someone can attest to the validity of the method. If so, any thick fabric should do somewhat the same as the STO's shroud damper, shouldn't it?