-It is forbidden to dance during Sefira, as it will lead to too much Simcha in this period of mourning. (See Mogen Avraham 493:1 and Mishna Berura 4893:3) It is also forbidden to play or listen to musical instruments during Sefira. (See Aruch HaShulchan 493:2 and Shu”t Minchas Yitzchok Vol. 1 Siman 111)
-Though some people are lenient with regard to listening to music that isn’t live (such as on a CD), the majority of Poskim forbid that as well as the end result (the fact that it will lead to extra Simcha) is the same.
Even those who rely on the lenient Psak should refrain from listening to tapes or CD’s that contain very lively dance music. (See Igros Moshe Orach Chaim Vol. 1 Siman 166, Shu”t Yechaveh Da’as Vol. 6 Siman 34 and Shu”t Tzitz Eliezer Vol. 15 Siman 33:2)
-It is permissible to sing during Sefira. However, one should not sing lively songs that may lead to dancing. (Psak of Rav Shlomo Zalmen Auerbach Zatzal)
If one is passively listening to music with no intention to bring him/herself to Simcha (e.g. while in a doctor’s waiting room, in a store or in another public place where music is playing) it is permitted to remain there, as it isn’t his/her intention to listen to the music.
If one is exercising and needs music for the rhythm, it is permissible, as it isn’t being done for Simcha. (Psak of Rav Shmuel Felder Shlita, Dayan in Lakewood, NJ)
Listening to Cantorial recordings (Chazanus) where the musical accompaniment is only as background, and isn’t part of the music, is permitted.
Likewise, children’s story tapes that have musical background are permitted, even when there are adults present who will hear it. (See V’Yevarech Dovid Orach Chaim Siman 65)
If music is necessary to calm and/or control small children (e.g. a musical mobile for an infant, or even a music tape in the car on long trips) it is permitted. However, it should not be played very loud that people outside, who do not know the reason you are playing it, will hear it. (Psak of Rav Shmuel Felder Shlita)