I was under the impression that you needed to have a Voodoo card plunged in to use a sound card with the Mediator under OS 3.X as it steels some ram from it to work.
Other than that I can't think of any other suggestions, but I am not using a sound card with my mediator at the moment.
But as I purchased one for mine back in September but have not yet had time to try setting it up yet, as I have to first change over from using my BVision under CGX4 to using my Voodoo3 PCI under P96 and the process of setting up the screen modes under P96 is putting me off.