Thanks again for your help.
I think (fingers crossed) that I have solved the problem. I had updated from Windows 7 to Windows 10 and noted in some forums that there were audio problems with Windows 10. As a result, I reinstalled the driver for my external DAC (Resonessence Herus) and it is working well now.
As you suggest, I will try having my anti-virus ignore my media files on the NAS drive.
The Wi-Fi was working fine on other connections (WD TV Live, an LG Smart TV and an Onkyo receiver) although I intend to try an Ethernet connection as well.
I installed Media Center 21 on the laptop and accessed the media files on the NAS drive via Wi-fi. Does that make the laptop a client or a media server? Some of the metadata (cover art, titles, notes, etc.) were fine but a great many were not visible in MC 21. It makes me nervous (paranoid?) about updating to MC 21. The laptop is an old one with Windows Vista and I have considered updating that too.