Hi Joynal's,
Thanks for using the Apple Support Communities.
Have you updated your iOS system recently. Because since iOS 11.4, Siri's voice has changed from previous versions.
Apple does provide few different voices and languages, you can change it by following the instructions below.
Go to Settings > Siri & Search.
Tap Language to change the language that Siri uses for requests and responses.
Tap Siri Voice to change the gender or dialect that Siri speaks.*
Best Regards,
Juitse
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Resource for this issue
https://support.apple.com/HT208316