Classical music in Paris

St. Eustache near Les Halles has free organ concerts at 5:00 on Sundays. This church is the second largest in Paris and has one of the largest pipe organs in Europe. The concert is about 30 minutes long and the organist offers some commentary on the pieces he plays - in French. There are also concerts held regularly at St. Chappelle - not for free. A Google search should provide information about schedules and ticket prices.

Here's a thread from about a month ago on this subject, lots of suggestions:

https://community.ricksteves.com/travel-forum/france/paris-church-concerts

Janis, hello. I'm a huge classical music fan, too. When I visited Paris in 2012, I happened upon a Baroque concert (by a South Dakota State University group, which also performed liturgical dancing) at St. Germain des Pres on the Left Bank. St. Germain des Pres, as it turns out, is the oldest church in Paris, even older than Notre Dame, St. Chapelle and other famous churches. To me it's worth visiting St. Germain des Pres even if there is no concert.

Also, St. Sulpice Church, which is also in the Left Bank area, is famous for its organ concerts. Rick has written about Sulpice: https://www.ricksteves.com/watch-read-listen/read/articles/powerful-music-in-paris.

In addition to those, make sure you check out the venues and performance schedules at concert halls in Paris. I've never been to a concert hall performance in Paris, but I went to the opera in Vienna this summer, a first for me. Don't get scared by the idea. Try to book a ticket in advance. The websites for most major concert halls in Europe have English buttons that allow you to book and pay in advance. After you pay, they usually send a link to your email box to print out the ticket before you go.

There are hundreds and hundreds of concerts each week in Paris. You can find the listings here at the Officiel des Spectacles:
http://www.offi.fr/concerts

The listings are posted/published on paper, too, each week. I don't see an English version, but you can use google translate to get the idea. As you can see there are 686 classical concerts listed for the 11-day period. There are another 1,000+ of other genres of music. Hope you find the perfect concert.

I second the recommendation for St. Sulpice! If you will be there on a Sunday simply attend a morning service. The organist there is excellent and plays a full "set" after the service has concluded.
You'll have no shortage of music in Paris...it's everywhere, always, every afternoon and evening!

You don't say what your dates of travel are , but if you are in Paris at the appropriate time in the season , try to get tickets for The Paris Opera Ballet ( normally at the Palais Garnier ) Spectacular music and incredible dance as well , have a look https://youtu.be/i18Q2RY1tzc

The year-old Philharmonie de Paris, again you'll find listings for performances here in the link that someone provided above.

there are free classical concerts in several spots around Paris every week; the Madeleine has mid day concerts and is a common venue for visiting choir and musical groups. We have often used these free concerts as a way to see a new part of the city. https://janettravels.wordpress.com/2010/06/17/music-one-key-to-unlocking-paris/
There is a concert at St. Chapelle many evenings which lets you see the beautiful old church and its stained glass as well as listen to the 4 Seasons for similar chamber piece. Several churches have organ concerts including Notre Dame.

We usually pick up a Pariscope for 50 cents or so from a newsstand and look through the music listings for free concerts; they also have many modestly priced options.