yea that could be the issue, you have an MPEG2 layer 3 file not an MPEG1 layer 3 file. At the bottom of the post are the sample rate and bitrate combinations to look up in order to figure out what MPEG layer 3 format your file will end up as. So for example, if your original file was sampled at 44.1kHz and you encoded your mp3 at 32kbps, you would get an MPEG1 layer 3 file. But if your original file was only sampled at 16kHz and you encoded at 32kpbs, your file would end up being an MPEG2 layer 3 file.
I would point out that any mp3 reader properly supporting LAME would be able to read ANY mp3 file you made in Switch. But I am guessing your player doesn't have full LAME support, which isn't out of the ordinary by any means - most commercial grade mp3 hardware players primarily support fraunhofer mp3 files.
MPEG1 layer III samplerates(kHz): 32 44.1 48
bitrates(kbs): 32 40 48 56 64 80 96 112 128 160 192 224 256 320
MPEG2 layer III samplerates(kHz): 16 22.05 24
bitrates(kbs): 8 16 24 32 40 48 56 64 80 96 112 128 144 160
MPEG2.5 layer III samplerates(kHz): 8 11.025 12
bitrates(kbs): 8 16 24 32 40 48 56 64 80 96 112 128 144 160