HTC One (M8) for Windows
Save a Ringtone - HTC One (M8) for Windows
Save a Ringtone - HTC One® (M8) for Windows®
From the Start screen, swipe left to display the Apps list.
(located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap the message containing the ringtone or sound file.
Tap and hold the link (file name).
save as ringtone
Ensure the following is entered/configured as desired:
Make this my ringtone
Enabled when a check mark is present.
(located at the bottom).