I have a 3DS and I must say the volume is too low even with headphones
I have a PSP and I recently completed Birth by Sleep and I kinda enjoyed its soundtrack. So I transferred a track to SD card and ran it on 3DS music application and it was significantly lower than when I heard it on the PSP. Not to mention, the output sound quality is also a bit inferior to the PSP.
This is quite disappointing really. I hope Nintendo can fix these issues with a software update. I was quite excited when I saw the Dream Drop Distance trailer with its graphics but I'm not sure if it will deliver in the audio now...