The speaker and microphone of an MFP and DFP have been collectively called an Intercom.
However the intercom provides a number of services.
The Volume schemes for the Intercom are 'Sounds and Voice' (speech synthesis) and 'Coversation' aka an intercom session.
If you call 'Edit volume using LCD' then by default (as per intercom behaviour menu description) it will edit the volume of sounds and Voice messages.
If you start an intercom session (talk between DFP or MFP's), then the volume level is set to the Coversation scheme.
If then you call 'Edit volume using LCD' it will edit the conversation volume.
So to summarise using 'Edit volume using LCD' edits the current active volume scheme.
Viv.