I think it's an inflection thing. You unnaturally emphasize the word, pause a bit, and try to fight back conveying incredulousness with your facial expression.
Silly-but-true answer: When I'm reading stuff like that aloud and wish to both be silly and make a point, I just read it as "registeredtrademark", very fast.
Serious answer: You don't. You just leave it out in speech.
In Ren & Stimpy comics, Powdered Toast Man always referred to the product for which he was named as Powdered Toast® or Powdered Toast™. I always took that as a hint that he pronounced the marks somehow.
Why the downvotes for such a simple, factual statement that answered the question? If you do an internet search for the phrase "C in circle" you'll find it is a quite common way to describe this symbol. On the Wikipedia talk page, someone mentioned hearing this description in their school textbooks which were recorded in an audio version. Way back when I volunteered for Recording for the Blind (now Learning Ally) I recall that this was also the designated way to read this symbol.