Resolution isn't usually a big deal - unless you try to display in a resolution your monitor doesn't support.
Usually though, with sound and no video, you have selected the wrong video output.
A detailed AVWizard description for nisse is here...http://wiki.linuxmce.org/index.php/AVWizard_Step_by_Step
Using an ati card can be problematic but http://forum.linuxmce.org/index.php?topic=3931.0
posted by ramoth would help nisse when having the black screen issue.
A quick link for ati cards is here - http://wiki.linuxmce.org/index.php/Display_Drivers
In 7.10 you can have a 1080p monitor, say give me 1080p in the AVWizard, but the monitor only wants to display in 640x480.
This is where you get into modeline and all kinds of xorg.conf headaches.
EDID, mentioned in the earlier post for 7.10, helps to allow the monitor to display in native resolution.
*Note: In 7.10 I had to do a few things to get true 1080p out of my Aquos LCD.
In 8.10 testing my LCD xorg.conf was created just great!
When I chose 1080p in the AVWizard I got 1080p (not like in 7.10 where some extra work was needed).
There was no messing around in xorg.conf, LMCE & KDE fonts, etc.
I just had to add a couple of inputs and ruby codes for my model of LCD - sweet!
I hope this further explains my earlier answer.