Low Volume on 2din chinese with iGO 8

I set the volume to 100% in settings but the voice and warnings are very difficult to hear.
My 2din only controls the music volume.
What I should do?

Open your sys.txt with a text editor (like notepad) and see it contains these lines

[sound]
queue_length=200
ding=0 ;
sound_volume=255
voice_volume=255
sound_muted=0
voice_muted=0

If not, add them. If necessary, change present lines.