As Mediator uses a PCI graphics card as a buffer, you will be limited to a 10 MBit ethernet card, I can't remember if any soundcards needs the graphics card to function.
For the price of the Mediator board, you can get yourself a X-Surf ethernet (also 10 MBit) and a Toccata sound card; this route might even turn out cheaper, if you buy it through eBay.
Alternatively, you can get the Mediator, buy a cheap Virge graphics card and let it sit unused; then you should be able to use a 10/100 MBit ethernet and any PCI soundcard.
There will be no problems with using a CyberVision 64 in the Mediator. It's a good card, especially with the buildin monitorswitcher.