Many thanks to those of you that have replied and tried to find solutions, to summarize and recap:
I was trying to add a custom sound to the System Sound folder, it transpires that with Ventura, System folders are locked down, but adding a AIFF custom sound file to the user Sound folder works. I did not have the Sound folder, but creating the folder and adding the sound file meant I had a custom Mail sound.
However, on trying to add a custom Messengers file (mR4) either in the user sound folder of one named ringtones, this in not shown in the drop-down to change the sound. Currently adding custom ringtones to Ventura is still a mystery. I hope this is correct, if not please advise, again many thanks for all the help.