To make the change, follow the steps to update the voice name in your speech synthesis request to the supported neural voice names in chosen languages by 31 August 2024. Starting 1st September 2024, standard voices will no longer be supported, please use neural voices for your speech synthesis request, on cloud or on prem. Follow the steps in this link.
If you have any additional queries regarding this retirement, please use comments on this thread to ask your specific queries and we will try our best to answer those queries. Thank you