Years later I wanted to post the ultimate guide on how to fix audio problems on Windows 10 for the ASUS N55SF (might also work for the N55SL or similar models). Let's start off by saying there are two different levels to this problem, one concerning your MBR (Master Boot Record) and the other one which is more OS level.

First Level Problem
If you receive audio to headphones or at any time your audio icon looked different from the one below, skip to the "Second Level Problem".
This is when you don't get any sound at all (not even from headphones) and your speaker icon in the system tray constantly has a red X like this:
Click image for larger version. 

Name:	icon.jpg 
Views:	10 
Size:	2.3 KB 
ID:	172974
Error code 10: device cannot start

This problem occurs because you have installed Windows 10 on a GPT disk (you can check this on Disk Management: right-click your disk - usually Disk 0 - select Properties, Volumes tab, Partition style). Unfortunately the ASUS N55SF audio drivers are incompatible with GPT, so the only solution is wipe your disk clean and reinstall Windows on an MBR formatted disk (unless you have a Windows 7 partition on another disk and want to try my trick, though I don't recommend it). Here's the steps I suggest you to follow to make sure you install Windows on an MBR formatted disk:

  1. Burn your Windows 10 ISO on a DVD. Yes, burn. Using a USB stick (FAT-32) I could only manage to install Windows 10 on GPT. No matter how I formatted my stick with Rufus, there was no way Windows would let me proceed the installation on an MBR disk. Using the good old DVD method could possibly spare you hours of frustration. And don't worry if you don't have anymore a DVD drive installed in your laptop: I was able to install Windows through a USB-SATA cable connected to my external drive.
  2. We are going to completely wipe your disk, so make sure you backup everything you need before you proceed. Boot your machine from the DVD and let the procedure load. Once on the partitioning screen, delete all the present partitions until all you're left with is unallocated space.
  3. Now hit Shift+F10 to get the command line. Type "diskpart" and hit enter. Run the following commands:
    • list disk (as to take note of the disk number you want to install Windows on)
    • select disk # (where # is the number of the desired disk, usually 0)
    • clean
    • convert MBR
    • create partition primary
    • format fs=ntfs quick
    • assign
    • exit

  4. Now you can close your command line and hit "Refresh". Click on your freshly created primary partition and click "Next" to proceed with the Windows install.

Second Level Problem
Once Windows completes the install process it will reboot and your audio will immediately work with Windows' basic audio driver. This driver, along with several other drivers for Windows 10 (even those from the Realtek website) will give you a very low quality audio. Plus, as soon as Windows has internet access, it will update your audio driver to, which will completely break your speakers audio. The only driver that will give you high quality sound is the original driver for Windows 7 ( that you can download from the ASUS website. You'll know you have installed the correct audio driver as soon as the orange audio icon from Realtek will show up in the system tray. Unfortunately Windows prevents the installation of this driver in any way it can, so here's the steps to follow to install the proper driver:

  1. Go to Start, type "This PC", right click on it and select Properties, Hardware tab, Device Installation Settings. Select "No, (your device might now work as expected)", then hit Save Changes.
  2. Then follow Robbmeisterr's suggestion, I'll update it here: go to C:\Program Files\ and right-click on the Realtek folder. Select Properties, Security tab, Advanced, Change permissions, Disable inheritance, click on "Convert inherited permissions into explicit permissions on this object.", click on the SYSTEM line and then on Remove. Then tick "Replace all child object permissions entries with inheritable permissions entries from this object" and hit OK, then Yes. Do the same thing again if you have a Realtek folder on C:\Program Files (x86)\.
  3. Now go to Device Manager (type it on Start), under "Sound, video and game controllers", right-click on "Realtek High Definition Audio" and select Properties, go to the Driver tab and check the Driver Version. If it says 10.0.#####... (which corresponds with your Windows version) it means that it's the default Windows driver and now you can try to install the proper driver from the ASUS website. If it says or some other wrong driver version, see if you can Roll Back Driver. If so, it should revert to 10.0.#####.... If you can't rollback the driver try to install the latest driver from the Realtek website. After you install it and reboot the system, try to rollback it again and get the 10.0.#####... You can also try from Device Manager to uninstall it and hit Action, Scan for hardware changes. What matters is you install the correct on top of the 10.0.#####... (if you still have problems you can try running the installers from safe mode).

If after all this you've managed to install the correct driver (meaning the orange audio icon from Realtek shows in the system tray), but for some reason the ASUS Sonic Focus utility failed to install, you can go to C:\Program Files (x86)\Realtek\Audio\ASF3GUI and install it from there (beware SF vs SM).

Hope this saves some pain.