Reducing stress lessens a person's sensitivity to pain.
Evidence
Researchers played extended audiotapes to patients before they underwent surgery and afterward while they were recovering.
One tape consisted of conversation; the other consisted of music.
Those who listened only to the latter tape required less anesthesia during surgery and fewer painkillers afterward than those who listened only to the former tape.
Conclusion
Reducing stress lessens a person's sensitivity to pain.
Evaluate
There are a couple issues we could look at. There's a study with a difference between two groups, i.e. a Curious Fact
CURIOUS FACT: why did the ppl who listened to music need less anesthesia / painkillers than the ppl who listened to conversation?
AUTHOR'S EXPLANATION: the music lessened their sensitivity to pain, so they didn't need as much pain relief.
The argument is assuming that there are NOT alternate ways to explain why the music group needed less pain relief.
Additionally, there's a New Concept in the Conclusion: reducing stress?
What did any of the rest of this conversation have to do with "stress"? It was never mentioned in the evidence, so the author must be assuming something about it.
Given that she thinks that listening to music lessened sensitivity to pain, if she's concluding from that that reducing stress lessens sensitivity to pain, she must be thinking that listening to music reduces your stress.