It seems this sound issue was related to a bug with the installer where "%base-packages" was not added to the system configuration file.¹ After working around the installer issue as described in the manual,² I get sound every time I restart the machine.
I want to test for a few more days before closing this bug. 1. https://issues.guix.info/issue/35541 2. https://www.gnu.org/software/guix/manual/en/html_node/Guided-Graphical-Installation.html#Guided-Graphical-Installation --- https://sirgazil.bitbucket.io/
