I recently noticed that both the DQ77KB and DN2800MT don't seem to have a build-in speaker for BIOS-errors. I also could not find the standard 4 pin gnd-nc-nc-v+ connector that is usually used to connect a speaker like this:
However, I did found an "Internal Stereo Speakers Connector" on both motherboards, but besides the pinlayout being different and having two instead of one audio channel, the used connector (JS-1125-04, with mating plug JWT A2001H02-4P) is of a type that I only know from old Soundblaster audiocards, used to connect a cdrom player to the audio card.
Since I haven't been able to find speakers with that kind of connector and pinlayout, how are users supposed to listen to BIOS-errorcodes if you can't even get a speaker that can be connected to these motherboards?
I am sorry you are having problems with this system but let me help you.
As you mentioned, these boards do not have the BIOS speaker due to the form factor. In this case it is necessary to connect speakers to the output jack in order to get the beeps.
I already tried hooking up a headphone to the green output jack, no beeps, from the OS that is. I haven't tried forcing the BIOS to generate beeps, although I was expecting the single short beep for a successful POST, but didn't hear that either. As for the OS, it's generating beeps through the BIOS speaker (startup-/shutdown-tune and a couple of other things) and it works fine on motherboards with build-in speaker or standard BIOS-speaker connector. But with the DN2800MT, which is running IPFire (=Linux router distro) there's not a single beep coming from the speaker-output. If it matters, IPFire uses the beep command to generate the beeps.
Are the beeps supposed to come from all audio connectors? As in: the internal stereo speaker connector (without the possibility to actually attach speakers to it), the external speaker output and the hd-audio/ac'97 connector?
And I don't see how the form factor is relevant to this, there are plenty of mITX boards with a build-in speaker or standard speaker-connector.
At this point I am not completely sure if the motherboard can handle the IPFire for that function so I would recommend you testing it without it. I do can confirm that just attaching speakers the system will make the beep sound.
I just forced (well, tried to) the BIOS to beep, by disabling the IGP. The powerled started blinking, but still no beeps on the headphone hooked up to the speaker output and yes, the setting for BIOS error when not detecting a GPU is enabled. Also hooked it up to an amp (RP-U200), no beeps there either.
Motherboard: Beep, beeeeeeep, beeeeeeep..........beeeeeeep beeeeeeep.
After ordering a couple of 3W/4ohm speakers on Ebay, which look just like the old model BIOS-speaker (before those small buzzers where used), I hooked one up with a cable not found in a computer since.... a decade or so and now I can hear BIOS-beeps. Although they are not as loud as motherboards which have a build-in speaker/buzzer.
Unfortunately, the beep-script from the OS doesn't seem to be compatible with this board, not hearing any beeps generated by the script.
So I'm guessing the developers of this board changed something more than just routing the BIOS-beeps to the audio-controller, maybe they blocked access from the OS(?), but at least now I can hear errors from the BIOS, if there are any. That is, if the beeps are loud enough to be heard outside of a computer case, got the board now in front of me on the desk and already noticed the beeps are not as loud as the original way of BIOS-error reporting.
Not sure how, but now I'm getting beeps from the OS, just not through the internal speakers I hooked up. Instead I can now hear them using the green speaker output on the back. Maybe installing alsamixer and unmuting everything helped.
Still leaves the problem of the BIOS-beeps being of very low volume, motherboards with a standard buzzer soldered on them sound at least twice as loud, even when build into a case the beeps are a lot louder than with the 3W/4ohm speakers hooked up to the internal connector of the DN2800MT.