Do you mean the audio from the monitor's own speakers? Just trying to be absolutely clear. I don't use mine so whether this is part of the issue for me as well is unknown, but I am having the same issue with respect to the stupid error message. I'm not sure the error message has credibility, however. I'm still able to pop into games and get the desired resolution and refresh rates.
Are you running the latest drivers? If so, the first one sounds like a bad cable. I had the same issue with the pack-in cable that came with the monitor I bought. Buy a better, preferably VESA certified cable.
As for the second one, check Windows Sound settings, set the default audio device and disable others if necessary.