Hello @James Bower , Thanks for using Microsoft Q&A Platform.
For Text to Speech Avatar, usage is billed per second. I hope you might have already done this process to identify anomalies and unexpected changes in your cloud costs using Cost Management and Billing.
I recommend opening a support ticket in Azure portal to investigate the billing issue further. You can find more information on how to create a support request here: https://learn.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request
I hope this helps.
Regards,
Vasavi
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