Hello, back again

yes get it changed under warranty, £100 for a new unit, however a 'tech 2' can not check the rechargeable batteries condition which are inside the 'powersounder' Vauxhall may say they can

, the powersounder is located under the scuttle, drivers side ie open bonnet, back left hand side, remove windscreen wiper arms, a couple of screws and a few clips, then the big piece of black plastic can be removed,

bingo you are looking at the 'powersounder'. (Mr Haynes has it fitted under the nearside wing). You are correct, only the indicators flash, no sound.
I have owned 5 Omega's, three had this problem, all 3 had low miles for there years ie the car did not charge the 'powersounders' batteries for long periods, these old batteries had a memory, but because the were never charged for long periods there 'full charge' may only of been 75 or even 50% full.
The best is insist Vauxhall change the unit

best regards