asmixer software package provides AfterStep audio mixer, 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 asmixer
asmixer is installed in your system.
Make ensure the asmixer package were installed using the commands given below,
$ sudo dpkg-query -l | grep asmixer *
You will get with asmixer package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Sound |
| Package | asmixer |
| Version | 0.5-14+b1 [armhf], 0.5-14 [amd |
| Description | AfterStep audio mixer |
| Sub Section | Sound |