Muting post sounds should do it. Don't think there's another fix besides disabling CSM and going full UEFI.

I had a similar issue couple of beeps and code 43 and my system will boot normally , how it was explained to me by EVGA Teach support that it's not really an issue if you have CSM enabled your  motherboard  is letting you know that your Graphics card doesn't support CSM because it only support UEFI since it's a newer bios , in another words this is not an error code it's a massage that you Graphics card doesn't support CSM legacy boot .

I have the same issue. So weird

I did not have the 3090 when I started using my ASUS Z470 board, so I did not get any beeps.  But someone in the ASUS forum asked about the CSM setting, with is not normally enabled for a UEFI install, and it was enabled on my board also.  I disable it then so I never heard the beeps.
 
I might assume ASUS is leaving it enabled for compatibility with an OS other than Win 10..

This started on mine as well after installing the 3090 K|INGP|N. I multi-boot with Windows 7, 10 and Linux, so CSM or Legacy BIOS are a must-have option. I do not use pure UEFI on any system. The beeps are super annoying and obnoxious. There is no way to disable the Ampere GPU beeping. I placed a piece of electrical tape over the opening (hole) on top of the motherboard speaker and this has dramatically reduced the volume of the beep. It is just barely audible now and no longer annoying. Since the speaker housing and electrical tape are both black, it is not even noticeable that the tape was added.
 
It seems to be common on Ampere and not unique to EVGA. TUF RTX 3090 4 RAPID BEEPS ON BIOS STARTUP | ROG Forums