Sorry, but I don't know. The volume slider in Panel should work.
Except if the playback Zone has been changed, since Volume Control is Zone specific.
For example, with my local Workstation connected to my HTPC MC Server in Client/Server mode, using a browser on my Workstation I just opened Panel from my HTPC MC Server and started playback in the Zone "Here". Volume control works. Then I switched Panel to Zone "Workstation", which is the Zone of my local MC installation on my Workstation. Volume control in Panel then no longer works, because it is controlling Zone "Workstation" now, and playback is occurring in Zone "Here".
I think I am right in saying that the volume slider has no effect if the file is being sent to a DLNA rendering device - which should have its own volume control. Can't get the strange volume options no matter how I try??
Not true, at least if the DLNA Renderer is a MC installation.
For example, with my local Workstation running the local Library, and not connected to my HTPC MC Server in a Client/Server setup, my HTPC becomes visible on my Workstation as a Dynamic (DLNA) Zone, and my Workstation becomes visible on my HTPC as a Dynamic (DLNA) Zone. So all connections between the two installations are using DLNA, and not specifically MC's Media Network. Note for this to work Media Network does need to be turned on in both the HTPC and the Workstation, and DLNA settings turned on as well. Server, Renderer, and Controller on the HTPC. Renderer and Controller on the Workstation.
Now when I open Panel for the HTPC, I can select the "Workstation" Zone and start playback, at which time I have full volume control, and can Mute playback if I wish.
Then if I change the Zone in Panel to "Here", I lose volume control, again because playback is happening in the Zone "Workstation", while I am trying to control Zone "Here".
Simple really.
PS:
I don't have an independent (non-MC) DLNA Renderer that I can send audio to and test at the moment, but if I use a DLNA player on my phone (mconnectPlayer Lite), select some local (on the phone) tracks, and play them to MC on my Workstation via DLNA, I get full volume control and mute of the Workstation. In fact, if I change the volume on the phone, the volume slider in MC changes, and MC's Volume Protection works, so that I am prevented from just selecting 100% volume on the phone and blasting my neighbours with music. The slider moves on the phone, and on the Workstation, then drops back down to a lower volume according to Volume Protection rules. Neat.
Unfortunately, the phone DLNA Player doesn't appear in MC as a DLNA Renderer, even though it is a Renderer. If it did I could test playback the other way around and show that MC can volume control an independent DLNA Renderer volume.