Firejail's website describes a known problem with pulseaudio 7.0/8.0:

The srbchannel IPC mechanism, introduced in PulseAudio 6.0, was enabled by default in release 7.0. Many Linux users are reporting sound problems when running applications in Firejail sandbox. It affects among others Arch, Ubuntu 16.04 and Mint users. This problem was fixed PulseAudio version 9.0. The following configuration will mask the problem:

$ mkdir -p ~/.config/pulse
$ cd ~/.config/pulse
$ cp /etc/pulse/client.conf .
$ echo "enable-shm = no" >> client.conf

If you have problems with PulseAudio 9.x use the previous fix, or configure enable-memfd = yes in /etc/pulse/daemon.conf.

This fix worked for PulseAudio/Firejail problems I had on 16.04 that were similar to what you describe.

Type alsamixer to the terminal and using left and right arrow keys in to the Headphones selection if is muted (shows MM on above) press M to unmute and press the up arrow to volume up

Afterwards insert the following command to the terminal:

alsactl --file ~/.config/asound.state store

To store the settings into a file to your home folder. Then in order to restore it every time you login (is the best option I've tried) use this command:

echo "alsactl --file ~/.config/asound.state restore" >> ~/.bashrc

Update

Also the source of the problem, in my case is that the Front Panel uses AC'97 instead of IntelHD one, so setting the appropriate setting into BIOS settings also fixed the problem whithout the need to do the following fix above.

So have a look first whether your motherboards Supports the older AC'97 format as well alongside with IntelHD, especially if you custom-built your desktop computer.