As some applications are Linux-only, it is very handy to have linuxulator available. In short, I just followed linux browser install and added slack and viber with pulseaudio setup. Although pulseaudio setup contains linux browser install, it is the most important part. Once the browser was installed I copied the scripts to make slack and viber working. After download of .deb file, chroot to linux directory and install it.

cp <slack>.deb /compat/ubuntu
chroot /compat/ubuntu /bin/bash
dpkg -i <slack>.deb
apt-get install -f

On host, script is needed in PATH so it can be executed just like a normal application, so /usr/bin/slack looks like this:

#!/bin/sh

get_pa_sock_path()
{
  PA_SOCK_PATH=$(sockstat | awk -v me=$(whoami) -F'[ \t]+' '
    $1 == me && $2 == "pulseaudio" && $6 ~ /native/ {
      print $6;
      exit 0
    }'
  )
}

get_pa_sock_path
[ -S "$PA_SOCK_PATH" ] && export PULSE_SERVER=unix:$PA_SOCK_PATH

/compat/ubuntu/bin/slack $@

Next, we need to create /bin/slack inside ubuntu chroot:

#!/compat/ubuntu/bin/bash
#
# chrome wrapper script from patovm04:
# https://forums.freebsd.org/threads/linuxulator-how-to-run-google-chrome-linux-binary-on-freebsd.77559/
#
export SLACK_PATH="/usr/bin/slack"
export SLACK_WRAPPER="$(readlink -f "$0")"
export LD_LIBRARY_PATH=/usr/local/steam-utils/lib64/fakeudev
export LD_PRELOAD=/usr/local/steam-utils/lib64/webfix/webfix.so
export LIBGL_DRI3_DISABLE=1
exec -a "$0" "$SLACK_PATH" --no-sandbox --no-zygote --test-type --v=0 "$@"

Final change is to /usr/share/applications/slack.desktop inside ubuntu chroot:

[Desktop Entry]
Name=Slack
StartupWMClass=Slack
Comment=Slack Desktop
GenericName=Slack Client for Linux
Exec=/bin/slack %U
Icon=/usr/share/pixmaps/slack.png
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Network;InstantMessaging;
MimeType=x-scheme-handler/slack;

Note: Exec line is wrong by default for linuxulator environment. The executable needs to be our script /bin/slack, not the slack binary itself /usr/bin/slack.

The trick is to execute pulseaudio --daemonize on FreeBSD host before starting slack. I got audio and webcam working. Of course, viber setup is the same.