The Piccadilly trains seem to have an ambient noise sensor that sets the volume at the start, while on the C Stock, it can adjust as it speaks (but in quite noticeable volume steps).
It can certainly be very loud in both cases, but there's no reason to chop off the start of an announcement. The C Stock also has a manually triggered announcement to say that it is being held at a red signal, something drivers must say after a very short time indeed (10 seconds?).
Given how mainline railway drivers often say nothing when held, or certainly not for a long time, I do think 10 seconds is a little too short - but that's for another thread.