There are many games that use the .brstm format. Such as Fatal Frame IV, Castlevania Judgment, SSBB, and so on. So try WiiXplorer(Available on the Homebrew Browser) to check the files on your disc. If they are .brstm files, you can then use BCSM-GUI 1.5 to encode .mp3 files to .brstm and replace the ones you want.
Edit: Actually it seems it uses .dsp files. But I think there are converters for it.