Was wondering if you are still using R281, like you said? Quite a long time now and R282 is still the only driver available on Realtek and does not offer the HD Audio Manager. If you already downloaded R281 in the past and you never deleted the chrome download history or format you computer it still there, otherwise do this to re-download again.

They still keep old drivers, it's just not on the website for download, only latest is there, but those download links of old versions aren't dead and can be downloaded, when you download something there's a download link, that's how i found thanks to chrome download history, if you use chrome go to realtek website and download R282 like usual (32 or 64 bits windows), after download complete do (Ctrl+J) to open the chrome download history, there you can see the file you just downloaded and below the download link of it, now click right mouse button over the link and copy it, open a new tab and paste the link in the address bar, now change 0009 to 0008 and R282 to R281 and click enter to download, doing this you can download any old version.

Realtek hd audio manager windows 10