@James Troy Yes, you can use the Azure speech service TTS for personal and commercial purposes as long as you are using an Azure subscription/resource that is not running on free credits i.e your subscription should not be a student subscription or a subscription which uses the free initial credits.
As long as your resource uses the free or standard pricing tier you can use the TTS feature to provide audio in the required voice and language for your content. If you are using custom neural voices, then please ensure to follow the responsible AI principles to disclose the usage of custom neural voices. For all other scenarios, you can wish to include/not include the acknowledgement.
The charges for the usage of the service is based on pricing of the service as documented on the pricing page. No additional royalty charges will be added for usage of the same in your audio.
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