Volume control extension assignment to zones

[DrCWO]

Hello,
I am a bit frustrated. Trying to build a roon volume extension based on one of the may volume extensions around in git.
Anything seems to be OK but the volume control is NOT SHOWN in any of my zones.
I also implemented a source control. This shows up in the settings of the zones, I can activate it (assign it to the Zone) and it works. So please would someone be so nice to have a look at my code and give me an idea why the volume control is invisible.
Help would be appreciated :-)

One more hint: the functions av_getVolume and av_getMute are called and deliver appropriate values.

    av.volume_control = svc_volume_control.new_device({
        state: {
            display_name: "Volume",
            volume_type: "number",
            volume_min: 0,
            volume_max: 100,
            volume_value: av_getVolume(),
            volume_step: 1.0,
            is_muted: av_getMute()
        },
        set_volume: function (req, mode, newvol) {
            console.log("************** NEW Volume: " + newvol);
            ev_volume(newvol);
            req.send_complete("Success");
        },
        set_mute: function (req, action) {
            if (action == "on") {
                this.state.is_muted = true;
                console.log("************** MUTE");
                ev_mute(true);
            }
            else if (action == "off") {
                this.state.is_muted = false;
                console.log("*********** UNMUTE");
                ev_mute(false);
            }
            req.send_complete("Success");
        }
    });

    console.log ("***** INIT Source Control");
    av.source_control = svc_source_control.new_device({
        state: {
            display_name: "Power Off",
            supports_standby: true,
            status: "selected"
        },
        standby: function (req) {
            this.state.status = "standby";
        // console.log("******************* Sent power off command!");
        req.send_complete("Success");
        ev_standby("Standby");
        }
    });