Bielefeld has a lot to offer if you like to listen to concerts – a lot of them are even free:
5.4. (Saturday) 12.05 If you fancy a little break from your Saturday shopping, there is (pipe) organ music of Bach, Buxtehude and others at the Altstädter Nicolaikirche (near Alter Markt / coffee store). For a list of music at various churches in Bielefeld, have a look into the “Kirchenmusik in Bielefeld” brochure (which is publlished every 3 months, I usually take it from the Stadtbibliothek) or click on “Veranstaltungskalender” here
6.4. (Sunday) The Bielefelder Philharmoniker play several family concerts at the Assapheum in Bethel, starting at 3 p.m. You can buy cake and coffee there as well. The last one of the current season is in April:
13.4. (Sunday) from 7.30 p.m. The Forum Bielefeld   celebrates it’s 40th birtday with an indoor festival and four bands: THE GRAVELTONES (Wales/GB) SODA FABRIC (Tel Aviv- Israel), THE 1840s (Israel) and BUFFALO SUMMER (GB)19.4. (Saturday) You can expect all kinds of surprises at Murphy’s Karaoke night at the Heimat & Hafen pub on Stapenhorststrasse

Five of the April concerts at Bunker Ulmenwall   are free for anyone under 18, two of the “Dienstag unten” concerts are free for all:
22.4. (Tuesday) B.E.D
29.4. Laksa (a quartet from Bielefeld)
28.4. (Monday) 8 p.m. University Audimax (on the left side of the main building): Kammerkonzert (chamber concert) of the university orchestra, they will play J.S. Bach. They even fixed the dates for the 2016 spring concerts 😉
19.5. (Monday) 8 p.m. Schwanengesang (Schubert songs) at the Audimax of the University
Thanks PollyEster for all the great concert info!