I doubt that third party system kext will work when installed in this BigSur path /Library/Extensions/ Ībout Wifi if your Catalina mac uses AirPortBrcmNIC driver, then in my prelinkedkernel I embedded the IO80211Family.kext patched, so the stock BigSur Wifi I guess won't work in this case, for sound try some external USB or bluetooth speaker, for internal audio card MacPro requires a different AppleHDA.kext rather than the HighSierra one. Some machines might try different prelinkedkernel, I uploaded some but other requires USBOpenCoreAPFSloader3 or to boot with CMD+S. However that failed as the Big Sur copy of csrutil seems to want to be run under Big Sur's rather than Catalina's recovery partition. I tried the workaround suggested of copying Big Sur's csrutil so that it can be run under Catalina's recovery partition. However, I have had no luck booting into the recovery partition under Big Sur in order to disable authenticated-root to allow mounting a snapshot for replacing AppleHDA.kext and IO80211Family.kext in /System/Library/Extensions. I have confirmed that the Catalina Patcher copies of LegacyUSBInjector.kext, LegacyUSBVideoSupport.kext and even AppleIntelPIIXATA.kext can be installed with Kext-Droplet. Replacing pre-existing kernel extensions in /System/Library/Extensions seems to still be an unresolved issue. From what I have been able to puzzle out tonight, it looks like anything that you can place in /Library/Extensions, without a pre-existing copy being present in /System/Library/Extensions, can produce a usable prelinkedkernel using the Kext-Droplet application.

Big sur unsupported mac wifi fix