Being morphed into some modded creatures with annoying idle sounds rly troubles me and I don't want to either delete these sounds in general from the mods or to turn down the volume control of all mobs. I don't want some(or any) idle sounds as a morph if it is possible.

There now should be a config option in the latest Changeling release. Metamorph and Changeling are interchangeable; you can switch back to Metamorph at any time:

https://github.com/asanetargoss/Changeling/releases