How can I get PySoundfile to run on Google Cloud Platform flexible environment?

I have a simple application that I'd like to deploy to a GAE standard or flexible environment, and it requires the generation of simple audio files.

I have code that runs well locally (on the dev server) using PySoundfile, but I run into errors with the underlying libsndfile when the application is deployed.

Traceback (most recent call last):
  File "/env/local/lib/python2.7/site-packages/gunicorn/arbiter.py", line 578, in spawn_worker
    worker.init_process()
  File "/env/local/lib/python2.7/site-packages/gunicorn/workers/base.py", line 126, in init_process
    self.load_wsgi()
  File "/env/local/lib/python2.7/site-packages/gunicorn/workers/base.py", line 135, in load_wsgi
    self.wsgi = self.app.wsgi()
  File "/env/local/lib/python2.7/site-packages/gunicorn/app/base.py", line 67, in wsgi
    self.callable = self.load()
  File "/env/local/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 65, in load
    return self.load_wsgiapp()
  File "/env/local/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 52, in load_wsgiapp
    return util.import_app(self.app_uri)
  File "/env/local/lib/python2.7/site-packages/gunicorn/util.py", line 352, in import_app
    __import__(module)
  File "/home/vmagent/app/main.py", line 3, in <module>
    import tonegen
  File "/home/vmagent/app/tonegen.py", line 4, in <module>
    import soundfile as sf
  File "/env/local/lib/python2.7/site-packages/soundfile.py", line 267, in <module>
    _snd = _ffi.dlopen('sndfile')
  File "/env/local/lib/python2.7/site-packages/cffi/api.py", line 141, in dlopen
    lib, function_cache = _make_ffi_library(self, name, flags)
  File "/env/local/lib/python2.7/site-packages/cffi/api.py", line 802, in _make_ffi_library
    backendlib = _load_backend_lib(backend, libname, flags)
  File "/env/local/lib/python2.7/site-packages/cffi/api.py", line 797, in _load_backend_lib
    raise OSError(msg)
OSError: ctypes.util.find_library() did not manage to locate a library called 'sndfile'

I've also tried wave unsuccessfully.

In the flexible environment you could try to build and use a custom runtime in which you can add non-python dependencies, like OS libraries (libsndfile in your case and whatever else you may need).

Basically the same answer as in How oracle jdk can be configured in Google App Engine Flexible environment.