Learn Spanish (Spain): Ya he encendido el micrófono, pero todavía no tiene volumen. - I have already turned on the microphone, but it does not have volume yet.

These two phrases are natural opposites. Ya means "already" and is used with he encendido to show an action is done. Todavía no means "not yet" or "still not", showing that the expected result (having volume) hasn't happened yet.

I remember that encender changes to enciendo in the present tense. Why isn't there an ie in encendido?

You are thinking of the present tense stem change (from e to ie). Past participles like encendido don't use those present-tense stem changes. You simply take the infinitive encender, drop the -er, and add -ido.

How would I use the lesson's grammar (volver a) to say "I have turned the microphone on again" here?

To say you did the action again using the present perfect, you apply the tense to the verb volver. The past participle of volver is irregular (vuelto). So, you would replace he encendido with he vuelto a encender (I have returned to turn on).

Could I say tiene el volumen instead of just tiene volumen?