I have the same problem and I have tried the following setting in sys.txt without any success! Can anyone offer me some help please?

[sound]
;android_muting_device=0
;ding_muted=1
;### (1) read by dex file, 0 fix for Android 4.x
;android_muting_device=1
;auto_mute=1 ;(0)ICS/JB
;keep_alive_speaker= ;(0)
;speaker_resume_sec= ;(3)
;allow_no_buffer_return_on_zero_volume= ;(0)
;logarithmic_volume= ;(0)
;compressor_switches_on_ui= ;(0)
;enable_silence_playing_muting_device= ;(0)
;dynamic_volume_type=1
ding=1
tts_volume=255
alert_volume=255
csv_voice_enabled=1
dev_channels=2
ding_muted=1
ding_volume=0
lua_voice_enabled=1
mute_os_key=1
pause_mp3_sooner=1
queue_length=200
wheelsound=0
working=1


[android]
navigation_audio_stream=4