Hi Jim,
If I do an "apt-cache search libvolk" in the terminal on a Raspberry
Pi 3 running Raspbian Stretch I get the following:
pi@picam1:~ $ apt-cache search libvolk
libvolk1-bin - vector optimized runtime tools
libvolk1-dev - vector optimized function headers
libvolk1.3 - vector optimized functions
pi@picam1:~ $
Which is consistent with the upstream Debian package info:
https://packages.debian.org/search?keywords=libvolk&searchon=names&suite=stable§ion=all
Please make sure that you actually run Stretch and that you don't have
any third party repositories interfering.
It might be best to just start with a fresh SD card and see if that works.
Alex