I tried with:
[warning]
speedcam_enabled=1
speedcam_warning=1
speedcam_continuous_approach_beep_when_tonal=0
overspeed_min_speech_repeat_delay=-1
overspeed_min_tone_repeat_delay=-1
speedcam_maxdistance_from_road=10

[speedcam_category:0]
activated_spoken_type=none
approach_beep_distances=200
approach_beep_spoken_type=sound
approach_beep_sound="Speed camera"
overspeed_spoken_type=none
warn_distances="150:500,140:500,130:500,120:500,11 0:500,100:500,90:500,80:500,70:500,60:500,50:500,4 0:500,30:500,20:500,10:500"
min_frc=-1
use_road_speedlimit=1

[speedcam_category:1]
activated_spoken_type=none
approach_beep_distances=200
approach_beep_spoken_type=sound
approach_beep_sound="Speed camera"
overspeed_spoken_type=none
warn_distances="150:500,140:500,130:500,120:500,11 0:500,100:500,90:500,80:500,70:500,60:500,50:500,4 0:500,30:500,20:500,10:500"
min_frc=-1
use_road_speedlimit=1

But - there is no beep sound now. One more information - I have Volume/Voice always disabled, but this probably doesn't have anything with beep sound.
I also noticed that Ding slider went to mute after I exit IGO.