To me... the most effecient way to boost sound/music volume without using Moonshell to blow up your music to fuzz, or use Audacity to edit all your music rips/MP3's is to get a Fiio E6.
http://www.amazon.com/FiiO-Headphone-Amplifier-Micca-Cable/dp/B005JT8OT6
I too also had an issue with the 3DS sound being way too weak. So I looked into headphone amplifiers, and found the Fiio E5, only to learn that a new Fiio E6 just released. The E6 sporting a 10-12 hour rechargeable battery, and the new addition of EQ modes (adds more bass and etc...). The size of it is about twice as thick as a DS cart, and a little bit wider, so its very pocket-friendly. It compliments the 3DS perfectly by not only having a matching LED, but allows you to increase the volume of your 3DS music/sound to the point where it won't blow out your ears. It also has improved noise reduction over the E5 model.
This is the solution I came to to solve my own 3DS music woes, and I'm much, much more happier this way than editing my music files. I would say this is essential to the 3DS as screen protectors are, and I don't say that lightly as I think 99% of all NDS and 3DS accessories are just plain useless crap. What you get in the order is the essential unit and cords you'll need, and is the best/cheapest, most portable headphone amp that you can get, anywhere. The next best thing is $70 and is the size of a smart phone.
Obviously if you and your kids aren't adept to using headphones while gaming/music, then you don't need it, but if you yourself is into music, then you should atleast get one for yourself, and possibly share.
And to give you an idea of how precious it is to me, I don't buy any random crap on Amazon. I spend roughly a week or more researching any product I'm thinking about buying, and then I only buy it if I might think it may be an item that is only necessary. I never buy stuff that I don't plan on really using because I'm a bit cheap like that (I still use a 7 year old PC).
The included USB and audio cables are of high-ish quality. They look and feel really sturdy for "free" cables.