This is a good silly little bug, and thankfully easy to fix.
TomTom love to think that they know what you want more than you do, so during a software update they mess with some of the settings.
In their wisdom, what they have done here is set the "Warn when going faster than allowed" option to ON (box ticked), BUT they have set the sound effect used to "Silence". Brilliant idea eh?
And it gets better.... You ONLY get to see the menu page to change the sound effect used, when you first tick the box yourself, so if it's already ticked, you never know it's there!
So what you have to do is this...
Go to Settings / Safety Settings, press "Next" to get to the second screen, and UN-tick the "Warn when going faster than allowed" option.
Then press "Next" and "Done" to exit the settings.
Then go back in again in just the same way, and TICK that option once again.
Now, when you tap "Next" you should get the sound Effects selection page for "Warning sound above speed limit" and you should be able to select your preferred sound effect.
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If you're keen you can even add your own sounds by using MyTomTom's "Add community content" feature and add them in the "Warning sounds" section.