Yes and No, most of the info there is outdated.
To edit/make new theme I recommend YATA.
For changing themes, CHMM
And for music... well that's a bit of a longer story but you should be fine for now with that guide.
Ninja'd:Use BRSTM Converter to convert wav/mp3 to brstm and then use BRSTM2BCSTM to make it work for the 3DS. See here: https://gbatemp.net/threads/tutorial-how-to-make-bgm-bcstm-files-for-3ds-themes.377927/
Not sure about custom sounds, I think there is a CWAV tool around here somewhere. You can ask RetroKenesis, he should be able to tell you.
Ninja'd: The tool is called Custom Theme CWAVs. I've never used it, so I can't tell you how to use it.
To edit/make new theme I recommend YATA.
For changing themes, CHMM
And for music... well that's a bit of a longer story but you should be fine for now with that guide.
Ninja'd:Use BRSTM Converter to convert wav/mp3 to brstm and then use BRSTM2BCSTM to make it work for the 3DS. See here: https://gbatemp.net/threads/tutorial-how-to-make-bgm-bcstm-files-for-3ds-themes.377927/
Not sure about custom sounds, I think there is a CWAV tool around here somewhere. You can ask RetroKenesis, he should be able to tell you.
Ninja'd: The tool is called Custom Theme CWAVs. I've never used it, so I can't tell you how to use it.