I finally figured out how to put custom ringtones on my HTC Trophy with the Mango update...WP7.5, V7.1.7720.
Find the ringtone you want. I use phonezoo.com to make my ringtones from a song.
Put the ringtone in Mymusic (or the folder that Zune uses for your Collection)
Open Zune and plug your phone into your computer
Go to Collection and find the ringtone
Right Click on the ringtone and select "Edit"
An Edit Song Info box pops up.
In the Genre box, type the word Ringtone.
Drag the ringtone to your phone
Go to your phone setting and Ringtones & Sounds
You'll find the new ringtone on your list and ready to use
Hip Hip Hooray!