haven´t had te same proplem as you guys but I was able to re-create it:

So let´s start!

First issue: The no sound with HDMI thingy.
I played around with some settings and noticed that I too lost sound entirely when I switched in my Xbox system settings sound output to "Digital Stereo". Everything went mute. What I recommend is that you switch it to Dolby Digital 5.1 . That works for me with the hdtv speakers aswell as my Sony Dav-is10 hometheater with optical cable.

Second issue:  The no hdtv resolutions with HDMI, only pc resolutions

The issue maybe your hdtv. The Xbox 360 and the tv do this thing, like a handshake that verifies what kind of monitor is attached to the console. If you have multiple HDMI ports, try those. The one with NO DVI support is your best bet on this one. Even on Xbox support center they recommend that you use a HDMI port that hasn´t got a DVI option.  If you don´t have mutiple HDMI ports, then your proplem maybe that your HDTV:s HDMI ports are 1.0 or 1.1 version.  The Xbox 360 supports 1.2 version, and you may need to buy a newer tv.

Hope this was helpful to some =)

yeah i'm having sound problems. i have to constantly turn off my console and tv and reboot them up

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Xbox 360 Elite on 40" Toshiba 40RV525U 1080p LCD HDTV via HDMI with Comcast HD and Sony BDP-BX1 Blu-ray Player

@Zekeban: I have testet all 3 HDMI Ports of my Samsung LE-46M86BD and even my Bedroom Samsung LCD does have the same Problem. Before I downloaded the last System Update everthing worked fine with my HDMI Connection. I still think the issue is, that I downloaded the Update while I was playing via VGA Connection on my PC Monitor, because my Gilfriend blocked the Big Screen. And now my Xbox thinks i'm always connected via VGA.

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Mein Xbox LIVE Space | NCC No'Clan Community

My 360 has suddenly developed a very similar problem, though it's not through HDMI, it's through digital optical. Previously got 5.1 output fine, now only a subwoofer signal - all other channels are mute. If I switch the console to Stereo, it works. Have Microsoft given any indication they are working on this problem? I use my 360 as my DVD players, so I effectively now have a useless surround system.

Have you tried using another optical cable?
Are you using HDMI and that separate audio adapter for audio? If so, have you tried using the optical output from the component/composite AV plug?
How about changing your sound settings on your xbox? Does both 5.1 and the WMA pro setting output the Subwoofer channel?
 

Why are you all doing such weird stuff. If you want the sound back just turn the volume up or down. It works, but the real problem is the sound going away.

 sui785 wrote:

Why are you all doing such weird stuff. If you want the sound back just turn the volume up or down. It works, but the real problem is the sound going away.

How does that even help his problem?? Thats probably the useless answer a retarded Microsoft tech support employee would say.
Cranking up his volume would just increase the volume for the Subwoofer where as the other 5 speaker are not getting any signal at all.

I'm having this problem too. No audio over HDMI and it is listing the VGA resolutions. Hello Microsoft anyone home?

This is EXACTLY the issue I'm having as well shays10. Did you ever fix your problem?

 shays10 wrote:

I'm connected via HDMI and lost sound about 2 weeks ago. It didn't happen right after the first or second NXE update.
Sound is gone and GFX are low-res (640x480).

Is this the type of problem you guys ran into? I'm trying to figure out whether I'm having the same problem or maybe my cable is ***.
Did all of you have HD graphics with no sound?
Currently I'm having this problem every 4th or 5th reboot - no sound and no HD graphics.
When the sound is back, the graphics are ok.

Thanks in advance

Sorry, I'd meant to quote this in my above reply. Anyway, still having this problem shays10?

I'm havin this problem too, it came out of nowhere and now I have no audio when i'm playing. My xbox is connected via HDMI cable to panasonic plasma tv. Is there any solution? (sorry my bad english)

bump

has this problem been solved? i have the same problem. woked fine until one day when i started my xbox and there was no sound. i contacted support and we tried different things that didn't work so i had to send it on repairs. 3 weeks later i got it back but the problem still remains.

 DethBoxx wrote:

I'm having this problem too. No audio over HDMI and it is listing the VGA resolutions. Hello Microsoft anyone home?

For the record, I swapped an Elite that was in another room for the Arcade Jasper having the problem and HDMI works perfectly. It's definitely an issue with the Xbox itself.