I don't know how the welcome aboard announcement is played - it seems to come on randomly nowadays and is nothing to do with the guard (but see below). The other announcements I am pretty sure you might be able to play by presing some buttons somewhere. Certaintly on the pre-refurb 165s on Chiltern there was a panel with buttons which could read out various pre-recorded announcements.
As far as I am aware, it depends on the rolling stock and the CIS software installed on it.
It is likely that with the announcements without 'Wessex Trains' in them have a new version of the CIS than those without - the announcement files would be changed on the upgrade so as to cut out the 'Wessex Trains' bit.
Regarding how systems are operated, many are now based on GPS signals (the Desiros certainly are and have co-ordinates loaded into them for next station announcements) and others based on wheel revolutions (primarily tube stock at least which would not receive the signal). Also the Welcome announcements here are sparodic - they always seem to play leaving Waterloo but other stations are hit and miss and I guess it's based on a pattern loaded in). Also I have heard 'Can the guard please contact the driver' on a Desiro so the driver must also have some control.
I believe the same system applies with Electrostars. However there are a whole bunch of reasons Julie can announce why the train has come to a stop outside a station and therefore the system must have a manual control aspect to it.
The calling pattern can differ between routes even with the same destination so if the CIS is not routecode based (for the calling pattern) then it must require some entry to it. Saying that though, on a Desiro and Electrostar if the train has an abnormal calling pattern due to engineering work (certainly Windsor and Eton Riverside via Wimbledon, and a slow service from Victoria to Littlehampton via Worthing) then the CIS will not play at least until it is on a recognised route - on a journey from ECR-LIT with a stop at Burgess Hill, Julie suddenly started working at Worthing. So, based on that, I would say that the system has predefined patterns built in that cannot be changed.
Certainly, guards and drivers would not have the ability to change certain files (or disable parts of an announcement - although they can disable the lot), nor the order of the files as these are all preconfigured in the software - after all connections on the same train every day are the same - coming along the SWML for instance into London and the next station is CLJ always results in a 'Change here for Richmond, Twickenham, Staines, Windsor and Eton Riverside, London Victoria, East Croydon, London Gatwick Airport, Kensington Olympia, and Willesden Junction'. Coming from WAT this is 'Change here for London Victoria, East Croydon, London Gatwick Airport, Brighton, Kensington Olympia, and Willesden Junction'
I think there is a certain flexibility in it for guards/drivers to turn the system off and have control over stopping certain announcements - no good changing at Clapham for Richmond if the Windsor line is shut is there? - and also to replay certain things such as the calling pattern, and specials (these are also automatic - unless controlled by the driver (which I'd doubt) - as the guard has been checking tickets when they've started).
Also I believe there is a time element built in to these (after all Desiros know what time it is as they display it to you most of the journey

). The standard daytime (from WAT) announcement for SOA goes 'The next station is Southampton Airport Parkway. Change here for Swaythling, St Denys, and please change here for Southampton Airport. Customers travelling to the city centre and the docks should remain on this train and change at Southampton Central'. Obviously changing at SOU for the city centre at 2200 would not result in much shopping being undertaken and, indeed, the final part of that announcement does not play at that time.
Gee I do go on a bit don't I? 