Clarifying licensing status - kkm, January 2019:

Copyright (C) 2005 Linguistic Data Consortium
Copyright (C) 2005 Tony Robinson
Copyright (C) 2005 SoX AUTHORS

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 2 of the License, or (at your option) any later
version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA

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Below is the original "License" section from the package README file. Since the
licensing state of the program is not clear from this language, believe GPLv2 to
be the most suitable for this code, as it the license used by sox, parts of
which are used, and it is probably the most restrictive of all mentioned here.

YMMV regarding the code that is NOT listed below; it lacks any licensing
statements altogether. My goal here is to make sure the package source is always
distributed with the binary, and that you can have something to say if you are a
startup getting investment.  I've been in this boat, and having a clear
licensing statement for every software package is usually a requirement.

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5. License

Various portions of source code from Tony Robinson's "shorten-2.0"
package are used here by permission of Tony Robinson and SoftSound,
Inc. <http://www.softsound.com> -- these portions are found in the file
"shorten_x.c"; please note the copyright information in that file.  By
agreement with Tony Robinson and SoftSound, Inc, the Linguistic Data
Consortium (LDC) grants permission to copy and use this software for the
purpose of reading "shorten"-compressed speech data provided in NIST
SPHERE file format by the LDC or others.  SoftSound provides useful
tools for audio compression and other signal processing tasks.

Other portions of source code (in particular the "writeRIFFHeader" and
"writeAIFFHeader" functions in "file_headers.c", and the "alaw2pcm"
conversion function) were adapted from the "SoX" package, a valuable
open-source tool maintained primarily by Chris Bagwell, with assistance
from many others (http://sox.sourceforge.net/).  We gratefully
acknowledge the value provided by all contributors to SoX; sph2pipe
would have been much harder to write without this resource.  We
recommend that you use SoX if you need to do sample-rate conversion on
audio data.