Answer: A.      In the Shulchan Oruch Yoreh Deah 391: A and B, it states that the Halocho is that a person in mourning must not attend joyous gatherings. B.      The Acharonim learned from this that one should not even hear musical instruments even if he is not attending a joyous gathering, and all the more so not play music himself. (Responsa Zera Emess, section B, 157, Responsa of Kfi Aharon, 52, Responsa of Maharam Shik 10:368, Responsa of Shleimas Chaim section 4:21, sefer Pnei Boruch 20:3). C.      If the mourner hears music from his friend’s house, he need not leave his house. (Shv”T Beis Meir 391, sefer Chayei Moshe section 4, P. 5137). D.      A mourner who hears music while waiting for a doctor or while using public transport, need not leave, but should intend not to enjoy it. (Nitai Gavriel, Aveilus section 1, 33:12). E.       If the mourner hears music at gym, one may be lenient that it is not for pleasure but for the sake of the activity at the gym. (See Kovetz Beis Hillel 50, P. 331.)