For me both
ffmpeg -ss 10 -t 6 -i input.mp3 output.mp3
or
ffmpeg -ss 10 -i input.mp3 -t 6 output.mp3
work OK, just 6 seconds of audio. Here’s the mplayer output (last line):
A: 5.8 (05.7) of 6.0 (06.0) 0.5%
Also
ffmpeg -ss 10 -to 16 -i input.mp3 output.mp3
work the same way. I use ffmpeg version 1.2.4. I guess your ffmpeg is somehow “broken” or the input file is somehow (report a bug in either case).
You may try the other answer with mp3cut from portforwardpodcast or
sox input.mp3 output.mp3 trim 10 6