First of all, you'll need to know that Usagi just support the preview of the BGM file, it doesn't covert the files to the needed format.
So, knowing this, you will need an audio file (the music for the background) and a converter tool, like BRSTM/BCSTM/BFSTM Conversion Tool (you can find it here in GBAtemp or GitHub, just search the name) to do the hard job for you.
Then, when you have the converted file (that need to be in the .bcstm format) you can rename it to "BGM.bcstm", put it in the same folder as the "body_LZ.bin" and the "info.smdh" an reload to test it. I didn't figure how to put song in my themes for the 3ds with any other way, and for now you can just test them in Usagi.