I got the sound working on my new system (Dell Precision 3640 Tower) with Gentoo after following the steps in Getting Sound (ALSA) Working on Gentoo. However, I couldn't get through from any of VirtualBox clients (Window$10, Manjaro Linux, or Ubuntu)
Some advices on the internet were to change the settings for Host Audio Driver and Audio Controller options on the VirtualBox host side. I tried all combination but couldn't get it working.
The information in this site is the result of my researches on the Internet and of my experiences. This information below is solely used for my purpose and may not be suitable for others.
PulseAudio is a general purpose sound server intended to run as a middleware between applications and ALSA. I never needed to install PulseAudio to get the sound working before, so I followed the Gentoo Wiki: PulseAudio.
Kernel Update & Installation:
The Gentoo Wiki: PulseAudio shows which kernel options to turn on using make menuconfig and recompiling the kernel.
# cd /usr/src/linux
# make menuconfig
# make && make modules_install
# make install
# grub-mkconfig -o /boot/grub/grub.cfg
After rebooting, make sure that the kernel version is updated by uname -r:
#5 SMP Fri Duc 18 14:46:23 EST 2020
Then, install pulseaudio:
# emerge --ask media-sound/pulse
Once pulseaudio is installed, enable the system to support pulseaudio for some applications by updating /etc/portage/make.conf:
Then update the system:
# emerge --ask --changed-use --deep @world
ALSA needs to be configured to output to PulseAudio by default:
# emerge --ask media-plugins/alsa-plugins
After following the above steps, the next time VirtualBox is started, the sound should be enabled from a client OS.