Where are system speech voices stored in MacOS Monterey?

The default voices in Monterey are stored in /System/Library/SpeechBase/Voices/. I used **System Preferences > Accessibility > System Voice > Customize...** to install a few more US, UK, and novelty voices, but I can't find where they are stored. Prior to Monterey, the voices resided in /System/Library/Speech/Voices/, which still exists but is empty. The /System/Library/SpeechBase/ directory is apparently new with Monterey, but it does not contain the additional voices I installed. Where are these voices being stored?