שמיעת מוזיקה בימי ספירת העומר

During the period of ספירה there was a plague that killed רבי עקיבא's 24,000 תלמידים. Additionally the Crusades and שר"י Chmielnicki's massacres took place during that same period of the year. As such, there is a מנהג to decrease שמחה, to which end, we do not listen to music played with instruments, even one person listening alone.

Since the principle of the custom is decreasing שמחה, music played for non-שמחה purposes, such as practice or פרנסה (where there is no significant enjoyment involved) is allowed. The same would apply to listening to music during work, driving or exercise in order to keep awake or maximize productivity.

At the same time, acapella music (vocals that are intending to resemble real music closely, such as where digitally modified or the like), although lacking instruments, provides the listener with a similar experience as real music, so acapella music would be no different; the מנהג not to listen to music applies there as well. The same would apply to background music, such as that which accompanies a story, or a ringtone (if the ringtone is musically meaningful).

Choral music, meaning a group of individuals singing together in multi-layer harmony or the like, is permitted, as this does not sound like real music, and, thereby, does not provide the listener with the identical musical experience as real music does.

If a person is in a state of discontent, they may listen to music to lift their spirits, however, they should not use not abuse this התר as an opening to listen to music excessively, i.e. more than what is necessary to improve their mood.

A child who is not developed enough yet to appreciate the ענין of אבלות may listen to music and even have music played for them by an adult, but one should make sure not to abuse this התר as well.

Hebrew Sources:

שולחן הלוי (הרב ישראל בעלסקי) פרק יג תשובה ו; מנהגי ישיבה תורה ודעת (ישיבה תורה ודעת תשעח) פרק יח הע' קנז; שלמי מועד (פסקי הרב שלמה זלמן אאוערבאך על מועדי השנה, נערך ע"י טוביה פריינד תשסד) פרק פה "שמיעת קלטות בימי הספירה"; אג"מ או"ח חלק א סי' קסו, שם חלק ג סי' פז, יו"ד סי' קלז; בין פסח לשבועות (הרב צבי כהן)  פרק ח סעיף א-ד, שם פרק טו סעיף ז; ערוה"ש או"ח תצג:ב

English Sources:

פסקי הלכה of Harav Yisroel Belsky vol. 1 (compiled by Moishe Dovid Lebovits, Israel Bookshop 2016) Chapter 32; פסקי הלכה of Harav Yisroel Belsky vol. 2 (compiled by Moishe Dovid Lebovits, Israel Bookshop 2022) Chapter 15; Rabbi Tuvia Posen's הלכה classes (self-published, 2022) pg. 392-395; http://matzav.com/p%E2%80%99sak-from-rav-yisroel-belsky-and-rav-shlomo-miller-on-acappella-music-during-sefirah-2/