I have the same issue. Some DVR recordings are fine, some will not play sound. This happens on my ipone and my apple laptop. Plays, with sound okay on set top box connected to TV. On demand is more ok but DRV recordings are what does not work the most. I have tried different browsers, clearing history, just about everything I can thing of. Only way to watch DVR shows is to wait until it is on demand and then watch that, not the recorded...?
I can take a deeper look into your audio issues. When streaming from your laptop an iPhone are you connected your home Wi-Fi network? I'd like to take a deeper into this for you. Please send me a private message by clicking on my name, "ComcastChe" then click private message me.
In the body of this message please verify the first and last name of the account holder, and the street address associated with your services.