Yeah, I'd suggest something like audacity then make sure you adjust the sample rate(I usually do 32000hz). Also make sure the length of the song isn't too long or else the file will be too big.
https://3ds.guide/faq
Q: How do I move to a new SD card at the end of this?
A: Just copy all the files to the new SD card.
Its in the official guide and I've done it myself..
But seeing as your followed that video guide, I guess you didn't read through the Faqs
I solved this issue by making sure the sample rate was 32000Mhz or lower in an audio editor(audacity, vegas pro, etc) before converting it with that program.
Otherwise it would do the same thing.