Take a look at the -t and -ss arguments. They should do what you want.
-t duration
Restrict the transcoded/captured video
sequence to the duration specified in
seconds. hh:mm:ss[.xxx] syntax is also
supported.-ss position’
Seek to given time position in seconds.
hh:mm:ss[.xxx] syntax is also supported.
For example, ffmpeg -ss 30 -t 70 -i inputfile.mp3 -acodec copy outputfile.mp3 should do the trick for the range you mentioned (30s-100s).