For various reasons, Windows 7 is not supported on Kaby Lake processor-based systems by either Microsoft or Intel. In many cases, driver support will only be provided by Microsoft's compatibility drivers (read: graphics performance, etc. will be awful).
Still, if you really want to try (I can't blame you; I *definitely* prefer Windows 7's lack of tracking software (imagine, I'm using "lack" to indicate a good thing...)), you could try (I offer no guarantees; I haven't tried it myself) using this driver version: Download Realtek* High Definition Audio Driver for Intel® NUC Kits (it's for the previous generation NUCs).
Hope this helps,