If you can't find the files on your computer, you can't put them on the SD card. Try to find out where your ripping application outputs to and copy those files. --- Sent from my 3DS Now playing: Atelier Iris 3, The Legend of Zelda: Ocarina of Time 3D
There is a simple way of asking this, I know, but just to be absolutely sure, right click one of the files on your computer, then click "Properties". Next to "Type of file", what does it say? The 3DS can support: MPEG Layer 3 Audio Files (.mp3) MPEG-4 File Format (.mp4) MPEG-4 Audio File Format (.m4a) MPEG-2 Advanced Audio Coding File (.aac) 3GPP Audio/Video (.3gp)
I also believe, unless I'm mistaken, it can support the standard Microsoft Wave Sound Format (.wav), if not, then it can definitely support the 5 I mentioned above. The last 4, .mp4, .m4a, .aac, and .3gp all come under the Advanced Audio Coding format, or AAC. And in short, the 3DS only supports MP3 files, or AAC files.
Another thing to note is that there must not be more than 100 files in a single directory in your SD card. It will play them, but only 100 will be shown, so this isn't the issue you're having, since it's not displaying them at all. That's the only thing I can think of -- it's an unsupported format.
The 3ds cannot support the Microsoft format. I tried it with some songs on my friends' lap top and they would not even appear in the music player menu. --- Aquatic Ambiance: Now that's some serious make-out music.