Not being heard is no reason for silence.

[N’être pas écouté, ce n’est pas une raison pour se taire.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 2 “Cosette,” Book 8 “Cemeteries Take What is Given Them,” ch. 1 (2.8.1) (1862) [tr. Wilbour (1862)]
    (Source)

(Source (French)). Alternate translations:

Not to be heard is no reason why a man should hold his tongue.
[tr. Wraxall (1862)]

That one is not listened to is no reason for preserving silence.
[tr. Hapgood (1887)]

Not being heard is no reason for silence.
[tr. Wilbour/Fahnestock/MacAfee (1987)]

Not being listened to is no reason to stop talking.
[tr. Donougher (2013)]