Well that was an interesting experiment.
Set the alert to sound at the speed limit rather than 5mph above and sound set to on rather than alerts only. It still works but....
Had to turn volume down as the directions sound was far too loud. The alert volume was then so low it was sometimes difficult to hear. When I got home went into settings again and the guidance volume is set to maximum. I guess that if I turn that down to say halfway this may resolve that issue.
Also - and this is a big issue - I noticed that the alert will only sound once in each speed zone. By way of explanation, travelling on motorway the alert sounded when I exceeded 70mph. I slowed down below 70 then sped up again to 70+ but no alert sounded. Tried this several times with same result. After going through a 50 limit stretch (unable to test here due to traffic) back to 70 limit the alert sounded again the first time limit was exceeded. Back on to 'normal' roads with 30 limit the alert sounded once my speed dropped below 30 then crept back up to 30+.
So my conclusion is that the alert is not really fit for purpose. Shame