Apple - Siri's voice on macOS Sierra is not natural

I had the same problem but figured it out. First of all, it does take some time for the high quality voice to silently download in the background, and get enabled. But for me, weeks went by and the low-quality voice remained. Here's why:

It has to do with your App Store System Preferences. I had all the checkboxes under "Automatically Check for updates" disabled, including "Install System Data Files and security updates". (This is to avoid any surprise bills when I'm tethering with my phone). Well, it turns out, when this is disabled, it won't download the voices from Apple's CDN (example URL: http://swcdn.apple.com/content/downloads/06/40/031-72924/0p9g9iuul9uaojp42eatzq4 b3zr38r4dgw/CustomVoice_en_GB_martha.pkg)

I haven't narrowed it down to this one checkbox (I enabled all of them) but I'm pretty sure it's the culprit. After enabling these checkboxes, I toggled Siri off and on a couple of times and watched the console - finally it downloaded and installed the voices and placed them in System/Library/Speech/Voices . You can keep an eye on System/Library/System/Library/Receipts to see when it actually installs the voices. Mystery solved (for me anyway).

Apple really needs to add a warning message or an override when you enable Siri if it's not going to be able to provide the high quality voice. It took hours of real digging to figure this out.

As it turns out, Siri's enhanced voice quality doesn't come preinstalled on macOS Sierra (just like on iOS). After you install macOS Sierra, your Mac will start downloading Siri's enhanced voice, which is a ~1.35GB download.

If you're facing the same problem and you still can't hear Siri's enhanced voice, even after a few days of installing macOS Sierra (like what happened to me), I advise keeping your Mac plugged in and awake at night (or a couple of nights if you have a slow connection), the enhanced voice should download overnight when you're not using your Mac.

There is also a thread on MacRumors forums regarding this issue (http://forums.macrumors.com/threads/low-quality-siri-voice.1981561/).

The things that worked for me is to uncheck the box "enable Siri " and check it again, it will ask you to set again "Hey Siri" however its quite simple Hope this solve your situation Regards