Does STK have a license?

Yes, we finally made something official for release 4.3.0. It is listed in the Stk class and a few other places in the distribution, but I'll repeat it here for clarity:

The Synthesis ToolKit in C++ (STK) is a set of open source audio
signal processing and algorithmic synthesis classes written in the
C++ programming language. STK was designed to facilitate rapid
development of music synthesis and audio processing software, with
an emphasis on cross-platform functionality, realtime control,
ease of use, and educational example code.  STK currently runs
with realtime support (audio and MIDI) on Linux, Macintosh OS X,
and Windows computer platforms. Generic, non-realtime support has
been tested under NeXTStep, Sun, and other platforms and should
work with any standard C++ compiler.

STK GitHub site: https://github.com/thestk/stk
STK WWW site: http://ccrma.stanford.edu/software/stk/

The Synthesis ToolKit in C++ (STK)
Copyright (c) 1995--2021 Perry R. Cook and Gary P. Scavone

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

Any person wishing to distribute modifications to the Software is
asked to send the modifications to the original developer so that they
can be incorporated into the canonical version.  This is, however, not
a binding provision of this license.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Why is my file sample rate wrong?

When the FileWvIn class loads a soundfile, it automatically sets its internal read increment based on the soundfile rate and the current STK sample rate. For example, if the current STK sample rate is 44100 Hz and the soundfile rate is 22050 Hz, the read increment, or rate, will be set to 0.5 and the file will be interpolated so that is sounds correct at 44100 Hz. For most cases, this works fine. However, consider the following example:

FileWvIn input( "infile" );                 // read an input soundfile
StkFloat sampleRate = input.getFileRate();
Stk::setSampleRate( sampleRate );           // set a new STK sample rate based on the file rate

With version 4.3 and higher of STK, the FileWvIn class will be notified of a sample rate change and it will automatically adjust its read rate accordingly. Previous versions of STK did not perform this change and thus, the read rate could end up being incorrect. If you do not want FileWvIn to perform this automatic adjustment, you can call the ignoreSampleRateChange() function for a given class instance.

Why does the sound I generated with STK sound like *&#@!?

If the resultant sound generated by an STK program sounds like noise (and you're not doing an MLS experiment), the problem is likely related to the byte "endianness" of your computer. By default, STK assumes "big endian" byte order. If you are working with STK classes on a PC (Windows or Linux), you must define the __LITTLE_ENDIAN__ preprocessor definition before compiling. If after reading this you realize you need to make this change, do not forget to recompile all STK classes from scratch.

Why do I get a Tk display error message?

The following error may be printed to your terminal window (depending on the version of the Tcl/Tk interpreter you are running) if you attempt to start an STK Tcl/Tk interface without the X Server first running:

Application initialization failed: this isn't a Tk applicationcouldn't connect to display ":0.0"

Simply start your X server and then try the command again.