NXDN having "better audio" is a matter of opinion. I have heard quite the opposite from many others, and I think Mototrbo (without encryption) has better audio than NXDN.
True, but it also has to do with the skill of the tech setting up the radios. Having run both side by side, NXDN sounded better.
After running an NexEdge trunked system for over 5 years now, I can tell you I've got them al tweaked out pretty well.
As for encryption, I have both encrypted and unencrypted talk groups and you can't tell the difference between then. The encryption/decryption process doesn't impact audio quality.
I've got an NX-900 mobile in my work truck running on my trunked system, 12.5KHz NexEdge trunking.
I've also go an NX-700 mobile in the work truck running on our public safety systems, 12.5KHz FM.
Both radios connected to identical speakers.
Same dispatchers, same microphones, same dispatch console system. Only difference is one goes into a digital trunked system (including encrypted talk groups) and the other one goes into a voted analog system using a combination of MTR-2000 repeaters/receivers and Quantar repeaters and receivers.
With the same dispatcher talking, the NXDN sounds a bit better to my ears. Like more dynamic range, fuller audio, but it's kind of splitting hairs they are so close.
So, hearsay is not a good comparison. Having run MotoTrbo, NexEdge and analog all side by side, the NXDN sounds better. If someone set up an NXDN system without fully understanding what they were doing, of course it might sound bad, it's what you'd expect. Some technicians didn't make the switch from analog only radios to digital radios very well, and there is some reluctance to do anything other than the absolute minimum to get them going. Spending time getting the audio tweaked out is key.
Of course some of that is the hand held/mobile radios.
A few years back I completed swapping out all the PD radios. Got rid of the MTS-2000 VHF, MCS-2000, XTL-2500's and the XTS-2500's, most portables going in to back up duty, and replaced them all with Kenwood radios.
Hand held radios are all NX-210's running in analog mode.
Mobiles are a mix of TK-5710's and NX-5700's.
Officers were really happy with the improvement in audio quality.
When compared side by side, Kenwood usually gets the nod for better audio on their radios.
But, opinions… yeah.