Read my message again. You would have two options:
First option: you modify the map's pk3 and give it a new name (i.e. fork the map) and include different music in it. That means you have to distribute the map to your clients and they have to keep it on their hard drive. If other server admins start doing this, clients will have to keep hundreds of megabytes of copies of maps just to change the background music. Proliferating different versions of the same map is just ugly.
Second option (I'm not even sure that would work for dlcache maps): you force your clients to download a pk3 with the music and mapinfos that you want to modify, and they override the map's mapinfo and default music. The problem with that is that, if the clients go to a different server after yours, they will be affected by your server's modification. That's not good. And as I said above, I'm not even sure it would work at all.
In any case, it results in annoyances for the clients, all this because you want them to hear something else. Not to mention that you are planning to distribute copyrighted content and put yourself AND your users at risk. That's just disrespectful and selfish all in all.
Anyway, you can try modifying your local pk3s as much as you want, it won't affect your clients. It does not work that way. And, I'll repeat it, you should not edit pk3s.