משנכנס אב ממעטים בשמחה
Based on this we have many ניהוגי אבילות during this period such as not eating meat, not bathing, etc.
Although there are a number of different reasons for not eating meat during the 9 days:
1. Aveilus - Meat is something that makes us happy (אין שמחה אלא בבשר ויין) and therefore in these days of mourning we abstain
2. זכר לקרבנות - We are supposed to remember that during this time the קרבנות were stopped.
The bottom line is that we are not eating meat so that we feel the aveilus of the churban. Unfortunately, many people take not eating meat during the 9 days in the wrong direction. Some people love it, as I once heard "and best of all, there's always ice cream or cheese cake for desert".
Those who feel that it is easy and great and they get to eat ice cream all the time, are missing the whole point. They may be doing the minhag but by Aveilus there is supposed to be a kiyum balev, you are supposed to feel something. These people feel no aveilus because they are not eating meat, in fact they are happy. This is a perversion of the minhag. The minhagim are supposed to help you feel aveilus, if they don't you should do something else that will help you feel the aveilus as well. Clearly, we cannot violate the minhag and eat meat even if we don't like meat, on the other hand, we shouldn't be eating gourmet milchig meals and enjoying ourselves either.
These days are supposed to be sad, eating gourmet milchig meals while not technically violating the minhag is certainly violating it's spirit.
The same thing applies to listening to music. The whole idea of "3 week CD's" (a cappella no music) is absolutely ridiculous. The whole point of not listening to music is so that you should feel the aveilus. It is clear that whoever listens to these "3 week cd's" enjoys them, otherwise why would they listen. This is a classic case of observing the letter of the law but not the spirit of the law or as the Ramban would say, a נבל ברשות התורה. Listening to these CD's clearly give people simcha the opposite of what we are supposed to be feeling at this time.