It increases volume of e.g. a diamond around ring (9)
I believe the answer is:
megaphone
'it increases volume' is the definition.
(a megaphone amplifies someone's voice)
'of e g a diamond around ring' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'around' indicates the central letters.
'ring' becomes 'phone' (synonyms).
The centre of 'diamond' is 'm'.
'e'+'g'+'a'+'m'='egam'
'egam' with letters rearranged gives 'mega'.
'mega'+'phone'='MEGAPHONE'
(Other definitions for megaphone that I've seen before include "Funnel-shaped voice amplifier" , "form of diplomacy?" , "Loudhailer" , "hearing aid" , "One projects sound" .)