Nothing very loud and important: out of tune (3,3)
I believe the answer is:
off key
'out of tune' is the definition.
(I've seen this before)
'nothing very loud and important' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'very loud' becomes 'ff' (musical abbreviation).
'and' says to put letters next to each other.
'important' becomes 'key' ('key' can be similar in meaning to 'important').
'o'+'ff'+'key'='OFF-KEY'
(Other definitions for off key that I've seen before include "Inappropriate" , "Musically, out of tune" , "Inaccurate in pitch" , "Dropped; hill" , "played badly" .)