There is an issue with Steam for Linux that seem to be persistent on machines with HDMI enabled displays or similar unused HD audio inputs.
Steam for Linux seems to use these controllers despite them being ignored by the system, which results in weirdly muted games while YouTube as well as any sound tests are loud and clear.
To fix this issue open sound settings and select analog controller or (for pulseaudio sound server) install pulseaudio volume control
sudo apt-get install pavucontrol
under “Output Devices” check if it tries to use the HD audio controller to output sound, go to “Configuration” and disable the HD audio controller profile so that analog audio will always be used.