jackd2 software package provides JACK Audio Connection Kit (server and example clients), you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install jackd2
jackd2 is installed in your system.
Make ensure the jackd2 package were installed using the commands given below,
$ sudo dpkg-query -l | grep jackd2 *
You will get with jackd2 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Sound |
| Package | jackd2 |
| Version | 1.9.10+20150825git1ed50c92~dfs |
| Description | JACK Audio Connection Kit (server and example clients) |
| Sub Section | Sound |