I installed iGO on many Samsung S3 with different OS ( JB, KK) root or no root.

Try to add in sys.txt :
[android]
;select audio stream
; 0-STREAM_VOICE_CALL, 1-STREAM_SYSTEM, 2-STREAM_RING, 3-STREAM_MUSIC, 4-STREAM_ALARM, 5-STREAM_NOTIFICATION
navigation_audio_stream=4

Force stop iGO and then restart.