As explained in answers to a related question [1], [2], you can use NirCmd, a small utility that does all kinds of stuff, including volume control, along with the Windows Task Scheduler.
Use nircmd.exe mutesysvolume 1 to mute the system volume, and use nircmd.exe mutesysvolume 0 if you want to schedule a job that unmutes it again in the morning.
Microsoft provides step by step documentation for using the task scheduler, but the basic process is simple:
- Open Task Scheduler
- Start the "Create New Task" wizard
- Follow the wizard prompts to schedule your job.