BCSAR files are archives for various audio related files.
Inside [FILE], there are usually .CSEQ, .CWSD, .CBNK, .CWAR and .CGRP files.
CSEQs (CTR Sequences) we have figured out, they obviously contain sequenced music and are also used for sound effects.
CWARs, which I'm pretty sure are CTR Wave Archives/WaveArcs, haven't been figured out to my knowledge.
We know they contain multiple CWAV files and are used as soundfonts and cries in the Pokemon games. I'll have to ask soneek for more on these files, as they're a planned part of my project.
The other formats I don't know what are, but I think CBNK files store sound modulation. Could be wrong, though.
There's not anything really worth digging into BCSARs for, unless your intention is to dig really deep, but if it is there is a tool for opening them.
I think it's 3DS Audio Ripper, as generic as it sounds. Speaking of which, it packs out any CWAV files it finds, which are basically just regular wave files. Used for move effects in the Pokemon games.