"Spoken Content" accessibility feature has stopped working

Hi there :) Hopefully someone has some ideas on this, because I'm stumped.

I have been using the "spoken content" accessibility feature for the past few months to read back documents in Pages & on Google Docs as a way to catch mistakes that I skim over while reading. I have had several versions of the Siri voice installed as the system voice, and have been using them all without issue.

Yesterday, when I selected some text and used the option+esc keyboard command to initiate spoken content, the little popup that shows playback controls for speed/pause/stop appeared, but no audio was generated.

At this point I did the following things to try and get it to work:

• Checked volume/audio settings -- I was able to hear music and system sounds with no problem, both through the laptop speakers and headphones.
• Reset core audio using terminal just in case. No change.
• Checked spoken content settings in the accessibility panel. Speak selection showed the checkmark as usual, and at this point the selected system voice was one of the US Siri voices. When I clicked "play" to hear the test phrase, nothing happened.
• Reset the computer.
• Tried to initiate spoken content again in Pages, then in Google Docs, then in Scrivener, then in notepad. Again, the control panel popped up as shown below, but the audio still did not generate.

• Re-opened spoken content settings in the accessibility panel. Speak selection still showed the checkmark, but now the system voice selection box was empty (see image below), though I had not removed any of the voices that I've had installed.

• Clicked on the system voice box to choose a new system voice, nothing happened. I no longer have the option to select any voice, or even to download a new one.
• Reset the computer again, still no change.

I have not recently deleted any files from my computer, but tried digging through my bin just in case something had been moved there automatically, but all that's in there is a handful of old gifs. The only other recent issue I've had is that my wifi is out, and I'm using my cell phone as a personal hot-spot for internet access on my laptop, but given that there is nothing I can find that suggests I need a high speed internet connection to use this accessibility feature, I can't imagine that it is related.

I'm running macOS Big Sur Version 11.4 on a 2017 Macbook Pro. I have not yet been able to update to 11.5 due to the aforementioned ongoing wifi outage, but will track down a place to do so if that turns out to be the problem.

If anyone has any suggestions beyond what I've already tried, that would be a huge help!

Thanks in advance.

-Cass