Are there any devices in device manager for which no drivers are installed ?
And again, are the CMI8738 onboard sound card drivers installed ? I ask because, as mentioned before, the MIDI port on your motherboard is likely connected to the CMI8738 . Additionally, I just checked and the CMI8738 drivers have a sub-device named MPU-401 .
Your situation would really make sense if the CMI8738 sound card drivers were not installed or had installation issues .
EDIT : An excerpt from the .inf file
MediaCategories="SYSTEM\CurrentControlSet\Control\MediaCategories"
WDM_MIDI="WDM-based Device"
WDM.UART.szPname="MPU-401"
WDM.FMSynth.szPname="FM Synthesizer"