How does the speaker system's headphone jack work? It sounds like it is being amplified by the speaker system's amplifier and the reason its loud enough.
Maybe there is some problem with the PC's sound hardware that the speaker system is covering up since it has a built in power amplfier. Based on what you've reported, that the volume level is low at both the front panel headphone jack and the rear speaker jack with more than one headphone, it does appear that there could be a hardware problem with the sound, assuming the volume levels are set up in the PC.
One thing to check, go to the Sound Panel, Recording section and make sure the System Mixer is set as the recording device. This is a long shot but worth checking/trying.
Also, the Intel web site lists a different file version of the RealTek for this motherboard. Generally, the PC or Motherboard vendor's drivers
are the preferred. Intel - 220.127.116.1102_PV.exe http://downloadcenter.intel.com/Deta...*&DownloadType
My old PC used an Intel DP35DP motherboard and I always used the Intel supplied sound driver
for it and didn't have any problems.