Nothing to do with WiFi - the O/S controls all activity and only programs that are loaded can operate the sound system.
Do you ever use Skype? If this program is loaded in the background by default (at boot time) it is theoretically possible for someone to call you if you have Auto-answer enabled. Also, if you were on the internet at the time that you adjourned to another room it is possible one of your previously visited websites could be running a video (advert or similar) in auto-play mode.
Otherwise I suggest the "hello" was issued by another device, not the computer. A mobile phone perhaps?
Computers are quite "clever" but are still dumb terminals that do not in fact have a life of their own.
It is good practice to shut down any program that is not actually needed if you are going to be away from the computer for an extended period of time.