Crop MP3 to first 30 seconds

I also recommend ffmpeg, but the command line suggested by John Boker has an unintended side effect: it re-encodes the file to the default bitrate (which is 64 kb/s in the version I have here at least). This might give your customers a false impression of the quality of your sound files, and it also takes longer to do.

Here's a command line that will slice to 30 seconds without transcoding:

ffmpeg -t 30 -i inputfile.mp3 -acodec copy outputfile.mp3

The -acodec switch tells ffmpeg to use the special "copy" codec which does not transcode. It is lightning fast.

NOTE: the command was updated based on comment from Oben Sonne

If you wish to REMOVE the first 30 seconds (and keep the remainder) then use this:

ffmpeg -ss 30 -i inputfile.mp3 -acodec copy outputfile.mp3