That's strange.
Like I said, feel free to upload the mp3 somewhere and I'll convert it to a working bgm.bcstm file. You can then compare files and hopefully figure out why you're having a problem.
If you could PM it to me that'd be much appreciated!Use bcwav for custom sound effects (don't remember where I got it though I could PM it). It converts .wavs to a usable format in Usagi but be wary of how much space you've used in the Theme Editor as it is limited.