I noticed that the quality of Siri voices is extremely better when my MacBook Pro M1 is plugged to charger. Once I unplug it the quality is the normal Siri voices. This is not to say that the voices are bad when the Mac is unplugged, the quality is really really great when the charger is connected. Just try yourself: connect the charger of your Mac and do a read all command while the charger is connected then disconnect and notice how different the voice is. I tried it with US Siri voices 4, 3, and 2. Not sure if this applies to other voices.
Now is there anything I can do to keep using the high quality voices while on battery? I get almost 17 Horus on a single charge so either my Mac is connected to power or not should not matter and affect the quality of the voices.