I had a similar problem prior to 1.6 update. To bypass this, I just set the alerts to 'on' temporarily and selected the 'none' option under 'sounds' menu and then turn off the alert again. Took care of the sound for me, but also removed visual notification (flashing red speed readout and possibly speed cam visuals) If you wish to keep these visuals only, simply restore the alert to on and as long as sounds are set to none, they should work OK.
V1.6 of the app seems to fix this audio issue (sort of), if you remove the speed alerts, the sounds also get muted. It also seems that the speed at which the alerts kick in has been addressed. It used to be pretty sensitive, but now (at least on my app v1.6) the alerts kick in when one goes over 5mph over the stated limit. That, and I don't seem to have any more Traffic Camera POIs or alerts listed anymore ( at least in my city in the US)
On a side note, for some reason it looks like they may have consolidated the Speed Limit & Speed Cameras options into one single 'Speed Limit' menu in 1.6, as I can't find that option anymore in the menus.
Hope this helps, even if just a little.
