Do the tones/screeches seem to sound like they're 'normal' or just random-noisy sounds?

Since this fault has 'recently appeared', I'd start by checking all the voltages on the soundboard, making sure that 5v=5v and 12v=12v etc, and make sure they're clean DC and not riddled with hum'n'junk.

After that, I wonder if you're missing an address pin (or decoder chip) which is placing the rom into a different sound bank, perhaps a bank that is used when this rom is in a different machine?

for example:
bank0='ripleys' sounds
bank1='junkyard' sounds

Otherwise maybe you're losing data pins on the rom data bus?

My next move would be to monitor the address/data pins while sounds are playing and looking for 1 or 2 pins that are doing absolutely nothing, meaning they're held in a state by a faulty address/data decoder chip.

Are all chip-selects active?

Hard where else to point at, without knowing or hearing the sounds and knowing how that particular soundboard works.

Always backup your roms BEFORE erasing them, oh well.....you may need to find some orig roms online, or another machine to copy from ;-)

MM.

Marty thanks for the response , all voltages are good , The screeching sounds etc don't just happen randomly they are addressed sounds just not correct ones.

I do have a backup ( always do ) of the original Roms but there is no way to back up the O/S , The new O/S gets written to a spare 27C512 and only used to flash upgrade the game O/S .
I haven't gone through checking data lines , addresses etc yet that's why I was asking the question here first just in case it was incompatibility issue with the new O/S 8 and this Game.
Biggest problem is I dont know what sounds should be where .