Here are the essential tools required to import and export user created music in Band Brothers, as well as some useful utilities.
Band Brothers Save Tool - This amazing program written by Zigg is what makes sharing Band Brothers music even possible. Unless you're using one of the homebrew DS programs like REIN, you MUST get this program to be able to swap out music in your game. The latest info and updates on the Save Tool can be found on zigg.com.
Extract the zip file into a folder, and run bbstool.exe to use it. Documentation on the software can be found in the \doc folder where you unzipped the program. The Save Tool is currently at version 0.3, which adds support for more kinds of .sav dumps.
Action Replay MAX PC App 1.4 - You should get this newer application if you were using software for an Action Replay that had firmware older than 1.04. This newer software is required for Action Replays at firmware level 1.04 and above.
Action Replay MAX Firmware 1.05 (US models) - This is currently the best, error free firmware level to use, and is recommended for swapping out music in Band Brothers.
The latest Action Replay firmware and PC software can be found on the official CodeJunkies Home Page.