You copy the sounds to the Notifications folder via the usb lead.
Once you have done that take the phone out of disk drive mode. Then pull down the settings, go to personalize, click on the sound at the bottom. If already on sound go to picture then back to sound so it refreshes. Then when setting email or sms, customs sounds will appear. Also appear from each app settings.
I have found that when in disk drive mode when the usb cable is connected that the sounds default again but soon as it is taken back out of this mode your custom sounds revert back.
Custom ringtones can be done from any mp3 on the phone via the player.
Hope this helps. Sure this would work on the other HTC One series but I havent tested it.