Ms. Caudill and I then went to a conference room to discuss music therapy with my husband, Don, and our friend Alexandra, who has engaged in music ministry. Music must be an especially effective form of therapy, Don supposed, because it directly expresses and creates emotions. Alexandra agreed. Singing in a choir for and with people impaired by dementia, she has witnessed elderly men and women who could not remember their own names recalling verbatim the words of beloved hymns.

“Music lights up neurons between the right and left hemispheres of the brain,” Ms. Caudill said. “It can also aid in neuroplasticity, helping the brain form new connections.” A stress reliever, music is used to recover speech, improve walking and assist in the retrieval of memories. Popular and classical melodies can be infinitely modified to meet various backgrounds and tastes.

[Read more about the use of music for mental health.]

Sometimes depressed patients are encouraged to compose new lyrics to a favorite song that can then convey their reactions to their condition. Ms. Caudill recorded a lymphoma patient singing her version of Shawn Mendes’s “In My Blood” and upon discharge gave her an MP3 of her new “anthem”: “Sometimes I feel I should give up, but I can’t. It isn’t in my blood.”

Others strum the reverie harp with Ms. Caudill or hum while she accompanies on guitar. She has drummed with relatives awaiting a family member’s medical decisions. One man asked for help choosing music for his own memorial; Ms. Caudill calls this legacy work and adds that it also involves facilitating life review, reminiscing and aiding in the clarification of values and beliefs through discussions of song lyrics.

In Oliver Sacks’s “Musicophilia: Tales of Music and the Brain,” the renowned neurologist drew on the “enormous and rapidly growing body of work on the neural underpinnings of musical perception and imagery” that started to evolve in the 1980s. It seeks to explain why those with brain injuries, epilepsy, Alzheimer’s, Parkinson’s, autism and strokes “may respond powerfully and specifically to music (and, sometimes, to little else).” With these misfortunes, as with cancer, rhythms and harmonies can deliver “Proustian mnemonics,” giving patients access to lost words and worlds. Or they can simply provide the solace of auditory pleasure when few other delights can be experienced or recalled.

Professionals like Ms. Caudill meet the spiritual, psychological or aesthetic needs of the afflicted by producing sounds testifying to the fact that beauty continues to exist in the world. Witnessing people in circumstances that have conspired to warp their sensory faculties and to reduce them to passivity, music therapists offer patients perhaps the only activity conceivable — that of listening — as a pathway to becoming sensate and thus incontestably attuned to the animating realization of still being alive and responsive.

Music has worked for me as a fail-safe sedative. During a fraught 17-day stay in the hospital because of post-surgical infections 10 years ago, Beethoven’s Cavatina furnished the only moments during which I was released from suffering. Don had brought me the CD, along with a portable disk player.