Audio recently stopped working on macbook pro 2014, Win7 x64

Hardware: Macbook Pro retina mid 2014 15" 2.8GHz 16GB RAM

Software: MacOS + bootcamp installed Windows 7 Professional N SP1 64 bit with latest updates attached to domain

Audio worked absolutely from installation for a year or so. Just recently speakers and headphone port stopped working and the SPDIF red LED light from the headphone socket is now constantly lit (didn't used to be lit at all). Possibly happened as a side effect of windows update, apple software update or NVIDIA driver update or application installation.

Which audio drivers should I have installed by Bootcamp support software for MBP mid 2014? Which actual audio hardware is present on this MBP - the Apple specs are vague on this point?

When scanning for hardware changes in device manager the same driver is always automatically reinstalled. There's no option to delete this. Updating the driver has no effect. The device is shown as working properly. there are no errors and there are playback devices for speakers, headphones and SPDIF but no audio is actually output.

• High Definition Audio
• Manufacturer Microsoft
• Location 0 (Internal High Definition Audio Bus)
• version 6.1.7601.17514
• date 19/11/2010

There was also a Microsoft system speaker device (doesn't get reinstalled when scanning for new hardware) and nvidia audio drivers (for HDMI audio output).

One additional symptom - plugging in headphones doesn't show the headphone playback device as then being a connected playback devices as I'd expect.

Is it possible the Bootcamp support software has failed to reinstall the audio drivers correctly? Can I get some feedback or a log from Bootcamp Support Software to see what's going on?

Has Windows in some way got locked into SPDIF output? Surely this can't be a purely hardware fault as MacOS still works fine?

Any further steps to diagnose this problem?

Some steps I've tried with no success:

• Checking all mixer and volume settings
• Rolling back recent software installations
• Looking at installed drivers in Device manager - all audio devices are shown as working correctly, scan for hardware changes doesn't find any extra sound hardware to configure. Updating driver by searching for drivers online doesn't find an updated driver.
• Using another USB audio device - this device works fine
• Rebooting into Macos - the built in speakers work fine
• Rebooting into Windows Safe mode - audio device doesn't seem to be installed (perhaps the driver isn't supported under safe mode).
• Resetting NVRAM (reboot with commmand option PR)
• Resetting SMC controller
• Running System restore - restore points fail to restore except for a few recent ones that don't fix the problem
• Uninstalling all audio drivers and rerunning bootcamp support software setup.exe (the slightly older download 5.1.5640 version that seems to be the only one that supports my hardware/windows 7 x64 combo)
• Rolling back NVIDIA driver to version supplied with bootcamp support software
• Running Intel chipset installers supplied with bootcamp support software
• Running boot camp setup assistant on MacOS to get more recent drivers - fails as doesn't support my combination of hardware and win7 x64 as documented on Apple support pages
• Running Apple Software update - no updates available for any drivers.
• Running Windows update - no updates available for any drivers