Hmm, mine seems to work the same on 1809 (screenshot attached).

As a test to see whether this is likely to be a settings issue in your current profile, could you try:

New Profile Test

This takes about 3 minutes, plus the time to test some sites.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the "Create a New Profile" button, then click Next. Assign a name like May2019, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

Is narration audible in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile.

Thank you for the troubleshooting steps, but when I double-checked my existing profile to make sure the narration audio issue was still present, the narration audio was suddenly working again. (Disclosure: I rebooted my PC for driver updates, including an audio driver update, after encountering the issue.) If I encounter this issue again, I'll follow the steps you provided.