Installing Windows on iMac (27-inch, Mid 2011)

Yes, from a physical DVD. If you have an ISO from Microsoft, you can burn it to a blank DVD-R SL media by right-clicking on the ISO and choosing Burn. It should look similar to

Use the lowest burn rate supported by the physical media.

Did You install the support software on windows?

edit: I don't really understand - did You manage to get windows working if You say there's no sound or trying to install windows? Can You boot into macOS?

Your 2011 iMac does not support EFI Boot properly. You need to use the built-in Optical drive and install W7/W8.1 using standard BIOS mode.

There is no official support for W10 on your Mac. However, you can upgrade W7/W8.1 to W10, using an in-place upgrade, which allows the W7/W8.1 drivers to continue to work under W10. Be aware that such W10 installations are not easy to repair, if you run into any issues.

euppi98 wrote:

I installed Windows 10 - the sound even works without drivers!

In Windows Device Manager, what are the available Audio devices?

euppi98 wrote:

But how now to install Windows on the GPT partition?

On a 2011 Mac, you cannot install using GPT/EFI and have your hardware work correctly. GPT/EFI is fully supported on late 2013 and later Macs.

SL is Single Layer (4.75GB), DL is Dual-Layer. You can test with DVD-RW, but I have seen issues with -RW discs in the past.

Please post back, if you run into any issues. On the older Macs with Superdrives, some of them have quit working properly and being out of warranty, no one wants to invest in addressing such issues.