Many many years late but if anyone is still looking for this homebrew, the link is in the forum titled Does anyone have this Nintendo DSi Homebrew: Pop'n DS by Yasu? created by the user above me
Main reason I am typing this is because I found way to get sound for bms/pms files besides the default files that are already in the homebrew (anpanman, etc.)
The gamebrew website states that only 16 wav can be played at the same time so if you replace all the wav files in the bms/pms folder with one single wav of the original song and edit the bms/pms file in notepad so that there is information for only that wav, there will be sound but unfortunately it will not be keysounded to each note if that is what you are looking for
Before you eject your microSD card and insert it back to your Nintendo console, check that the notepad information of each bms/pms file displays #BPM01: [insert bpm number] and #00001:0001 after the wav numbers otherwise there will be no sound. I don't know why it works, I just compared the notepad information of the bms/pms files that had working sound to the ones that do not have sound and noticed that information is what makes the sound work
If there is still no sound, the gamebrew website recommends for the wav file to be 8-bit mono with a sampling frequency that is a multiple of 11025Hz so you can try to import the wav file into Audacity and export the wav file back into the bms/pms file folder with those properties
The l2u file is for if you want the file information to display Japanese. To get the files, download the RetroGameFan Multi Cart Update v7.06 by retrogamefan. Then extract it and I am not sure if the system files are the same in each folder but I copied system.ank, system.fon, system.l2u, and system.u2l from the TTMenu folder in the DSTTi-Clone YSMenu folder to the BMDS folder on my Nintendo console
Image Attachment Descriptions:
1. Original folder that has not been edited (has 0001.wav, 0002.wav, 0003.wav, ...)
2. Original bms/pms file information that has not been edited in notepad yet (has #WAV01 0001.wav, #WAV02 0002.wav, ...)
3. Folder that has been edited, note: 0001.wav is the wav of the original song, not the original 0001.wav
4. What the wav section of the bms/pms file information looks like after it has been edited in notepad
5. Addition of the #BPM01 and #00001:0001. What comes after is part of the original file