If you drag a Voice Memo to the finder you can find it under Get Info > More Info. This is handy because it syncs the memos from the iPhone back to the Mac for direct comparison. You can also see the it being negotiated in the debug console for both Mac and iPhone or in the Bluetooth Explorer audio graph for the Mac.
It is highly unlikely to be related to the BT version itself. There have been no bandwidth improvements to classic Bluetooth for decades. And Apple is unlikely to use LE Audio since BT 4.0 didn't had the required bandwidth for even full duplex 8kHz.