I need to reinstall macOS after Windows install

I’m a PC technician, I don’t usually work on Apple devices

but sometimes I make an exception, this is one such occasion and I need help …

please !!!

I have helped this client in the past so when he again asked me for help I

agreed, he was unhappy with his Mac (apologies, I don’t know the exact model

apart from the fact it’s a 21” screen it says A1418 on the base) and wanted me to recommend a Windows

machine for him. I suggested that rather than spend £300-£500 on a new device,

for the price of a Windows 11 key and my time I could install Windows on his

Mac (I only know this is possible because a lady asked me to recover data from

her deceased father’s Mac’s only to find they were both running Windows 7 !).

Anyway, the installation and all of the Apps installed just fine and I was

about to return the machine to him when I realised there was no sound. I have

tried everything I would normally do but to no avail. After investigation

online it seems to be something that needs to be done within macOS as Windows

should have been installed alongside macOS, this I didn’t know and installed

Windows on a formatted drive.

So, I’ve now tried to reinstall macOS but it won’t work, first of all I chose

Restore from Time Machine Backup but there is nothing found, then I tried

Reinstall macOS High Sierra but I get the message “The recovery server could

not be contacted” (WiFi is connected). Next I clicked on Disk Utility, and

selected the Internal Drive, clicked Erase and selected APFS then Erase again,

I got the message “Process is complete, click Done to continue”, I did so and

tried the Reinstall macOS again but the only difference is that took a lot

longer to tell me “The recovery server could not be contacted”.

I did notice that in the Disk Utility there is a Partition option, but the

online help I found didn’t say anything about this.

Can anybody please explain what I need to do to get macOS reinstalled so I can

get Windows running with sound (using Boot Camp I believe) ?!?!? Or, of course, how I get the sound to work on a formatted drive & standard Windows installation !!!