Did you get rid of the previous drivers before installing the new card, preferably using
DDU or something similar to clean as much of the residue as possible? If not, do it now and reinstall the drivers. Even if you stayed with the same manufacturer, different settings will be made by the system according to the model.
Until now I had never heard about the GPU requiring any certain minimum of system RAM. That said, of course too little RAM will cause extra page file usage which may cause issues. 8 GB sounds like a reasonable amount, though.
My approach to memory has always been quite straightforward: If it fits, it should work. The motherboard manual should tell the supported speeds. At least the information should be available at their site. Since you already have quite some, getting the same speed RAM sticks should cause the least issues. If you have 2x4 sticks now, adding two more will double the power consumption of the memory slots. If you have one single 8 GB one, getting a pair for it is recommendable. If you end up upgrading the existing sticks, you won't see any visible difference using "faster" or "slower" sticks. But of course, if you can get faster sticks for the same as slower, take them. Ask for further advice here when you know your alternatives.
As for the sound issues, check your sound settings. Having done changes in your system may affect in strange places. In this case it might be that Windows has changed your microphone settings to the video card hdmi audio and the game is reading that information. If you're not using your monitor through hdmi and thus not using your hdmi audio, disabling it in the Device Manager and uninstalling the driver for it is good practice.
