Muted sound when treacherous fascist backed (3-3)

'muted sound' is the definition.
(eg sound made by a muted trumpet)

'treacherous fascist backed' is the wordplay.
'treacherous fascist' becomes 'hawhaw' (I can't justify this - if you can you should believe this answer much more).
'backed' says the letters should be written backwards.
'hawhaw' written backwards gives 'WAH-WAH'.

'when' acts as a link.