.bms file holds data for sequenced music in Pikmin 2. BMS files are similar to MIDI files, but have many more advanced features. Custom BMS files can be made with Flaaffy by Arookas (which can be found here), however Yoshi2's fork of Flaaffy is better suited to making BMS files for Pikmin 2. You can find that here.
List of BMS files
A full list of all the BMS files in Pikmin 2 can be found here.
A lot of BMS music in Pikmin 2 is interactive, meaning that parts of it change depending on what you do in the game. BMS files produced by Flaaffy do not support this by default, however they can be modified to be interactive. A tutorial to do this can be found here. This currently only applies to overworld music and will not work for caves.