For Garmin I can't find any info on their website or manuals about actual voice or sound alerts. They just say, "speed limits for most major roads".
From my quick research, it seems that garmin units only change the colour of the speed to a red background or a flashing red background.
I took a look at TomTom and they seem to have an audible alert that goes off when over the speed limit. They seem to be limited to 'some' major roads as well. Also it doesn't seem like you will be able to change the alert sound.
Hopefully some one else will chime in with some actual roadway experience with these newer units.
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Automotive Installer, A.E.A.T. Ontario Cert.
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