Output volume control for build-in speaker of DS-2CD2386G2-ISU/SL

[Dreamliner]

I got a HIKVISION DS-2CD2386G2-ISU/SL camera with a build-in speaker. Frimware 5.5.160
But the output volume of this speaker cannot be changed from Web-interface: Configuration/Video-Audio/Audio.
Any idea how to do it?
Thanks