It seems that in practice, according to the lenient view, we should divide all songs into three categories: 1) joyous songs, like those played at weddings; 2) songs that are neither especially joyous nor especially sad, which includes most contemporary music and most classical compositions; 3) sad songs, like those played or sung when mourning a death or the destruction of the Temple.
From the beginning of the Three Weeks, one should refrain from listening to the first category of music – joyous songs. Starting from the first of Av, one should refrain from listening to the middle category as well, and listen only to sad songs, the third category. It also seems to me that when one listens to loud music, even if it is a neutral song, the force of the sound makes it more festive and practically transforms it into a joyous song. Thus, one may not listen to loud music even if it is the type of music that is permitted during the Three Weeks.
Furthermore, it seems that one may not attend a concert featuring sad music (requiems) during the Three Weeks. Even though the music is mournful, concerts are festive and joyous events, as evidenced by the fact that people usually dress up for them. However, it would seem that one may play a sad tune in commemoration of Jerusalem at a cultural event, even during the Nine Days (based on Shabbat 151a).
