Complain when volume is not available (5,3)
I believe the answer is:
sound off
'complain' is the definition.
(synonyms)
'volume is not available' is the wordplay.
'volume' becomes 'sound' (sound levels).
'is not available' becomes 'off' (eg the power might be off).
'sound'+'off'='SOUND OFF'
'when' acts as a link.
(Other definitions for sound off that I've seen before include "keeping the volume right down?" , "heard but not seen by audience" , "Speak angrily" , "Complain" , "Rant" .)