My first Sound blaster card, ISA 16 bit, I made a copper plate shield for, and put it at the far end of the ISA card slots, so the shield was only on the component side, the case itself making up the rest of the shield. Lost the ISA slot next to it though, as I used the blank to mount the copper foil, with a cardboard layer each side for isolation, and used a copper braid to attach it to the case. Dropped the audible RF noise a lot on the card though.
With on motherboard you are SOL though, as they typically either did not worry about any noise induced into the audio, or costed out the decent RF filtering during manufacture, replacing the RF chokes and the bulk capacitors with zero ohm links and nominal chip ceramic capacitors. Some boards you probably could reinstate the capacitors and chokes and improve things, but the best is to use an external DAc of sorts. Preferablu one that has a Toslink connection , as that provides really good RF isolation as the signal goes via a non copper path, though there you are also going to have design issues if the power supply for the DAC there is noisy.