The same firmware [unit software] is used for all 7x0. The TTS function in 710 is blocked by the standard region 16 [logo/splash/start screen] recognising the unit as a 710 and disabling the TTS function which means that if any TTS voices are loaded to the unit's Voice folder they are simply ignored.
The logo supplied to the AU version of 7x0 has a different splash screen appearance to the standard US/EU logo as shown above.
But there is another very important difference if you're the owner of a 710. Garmin didn't 'neuter' TTS for 710 in the AU 7x0 logo, probably because 710 was never sold here and so it wasn't considered necessary to do so. So by flashing the AU logo to 710 you can enable TTS. When connecting to WebUpdater you'll then be offered all applicable TTS voices.
What you can do:
You can flash 3 different 'versions' of the AU logo. The standard AU logo with the Sensis 'WhereiS' start screen [AU_reg_16], or a modified AU in which i've replaced the start images with the usual 'Garmin' logo [AU_Mod_16], see images above. Both will identify in the unit's Inventory page 'Splash' entry as 'D0826-01 2.00 nuvi-wide Australia Splash Scre Pass' whereas the US/EU splash identifies as 'D0825-00 2.00 nuvi-wide Splash Screen Pass [however, if you've ever changed the start-up pic to one from the nuvi's Picture Viewer i think you'd have 'Fail' instead of 'Pass' until the day the nuvi dies]. If you'd prefer to not have mention of 'The Great Southern Land' in your Inventory page, you can flash the 3rd file [AU_HEXmod_16] a hex modified version of the AU file with the Garmin splash which will identify as 'D0826-01 2.00 nuvi-wide 710EU TTS Splash Scre Fail'. The 'Fail' is because i haven't calculated the checksum in hex [good reason: i don't know how] [Edit: i do now, thanks to Loly, see Post #2]
and anyway it doesn't matter 'cos Pass or Fail the start screen still works fine and you'd never know unless you look at the hidden menu's Inventory page [Press Battery symbol on main screen 'Where To? View Map' for ~6 secs>Next>Start Test>More].
One of our helpful Software Experts might like to post to tell us how to do that so that 'Pass' appears instead.
How to actually do it:
First, a reminder that any flashing can carry risks. I have flashed all 3 of these logos to a 710 without problem but i can't and won't guarantee that you will not experience unsatisfactory results. If you follow the instructions carefully and completely there should be no problem, the worst that is likely is for your unit to start without a splash picture and then nonvol rgn16 will need to be re-flashed, but you'd need to royally stuff up to get that problem. In the unlikely event of you having problems, post with the details and i'll do my best to guide you thru.