I experience something similar with my desktop. In my case the loud, singular pop is definitely emanating from the speakers and occurs at both boot-up or shutdown. Volume and mute have no control over it.
I had always assumed this anomaly was either sound card driver
related or perhaps something caused by the sound card receiving or losing electrical power during the boot up or shutdown event, or in the OP's case, the sleep event.
In either case, I have experienced it across a couple of different desktops to the point where I consider it harmless but slightly annoying.