Voice of the Chain Master only allows you to command them
Conjuring
Summon Greater Demon has a range of 60 feet and says
The demon appears in an unoccupied space you can see within range
While the spell does not explicitly use the word target, conjuration spells that have the conjured creature appear in a space in range when cast are considered targeting that space. It is the same for Conjure Elemental.
You familiar also cannot cast the spell for you, and you cannot cast the spell through it -- the find familiar spell only allows you to deliver a spell with range of touch through your familiar, which neither of these spells is.
That means you will need line of effect from yourself to the space where the demon or elemental is going to appear, Voice of the Chain Master, or not.
(And you cannot cast either of these spells while under the Effect of Meld into Stone, because you can only cast spells on yourself when melded into stone, and neither of these spells is being cast on yourself.)
Commanding
The feature text of Voice of the Chain Master says:
while perceiving through your familiar’s senses, you can also speak through your familiar in your own voice, even if your familiar is normally incapable of speech.
Once you have summoned the demon, you can use your action to perceive through the familiar's senses and you speak through it, in your own voice. So it is not the familiar speaking, and it is not a mimicry, it is you speaking, with your own voice, and you can command the demon or elemental with it.
Lastly, because you brought up the name Voice of the Chain Master: the names of game features are figurative, you should not change the reading of the rules text based on them.
