It's always best to kind of narrow potential hardware problems down.

It could be that the volume in the system configuration is down low and/or off?
It could be the client? Maybe deleting & reinstalling the client could fix it?
Maybe it's a new issue that hasn't been brought to light?

It's not the HDMI cable because you're getting sound at the log in screen. Although to make sure of this try using the cable on a different source (different TV/screen) to be certain that it is not the cable.
Try putting the cable in a different HMDI port on the screen?