@Benoit Rivaux Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
The Azure AI Speech service does offer 0.5 million characters free per month for the Neural Text to Speech feature under the Free (F0) tier. See the below screenshot:
More info here.
If you are Free (F0) tier, You need to ensure that you haven’t exceeded the limit of 0.5 million characters in a month. Once the limit is reached, you may be charged for additional usage
If you are on Standard Tier (S0), below is the pricing:
Also note that for Text to Speech: usage is billed per character. Check the definition of character in the pricing note.
Hope this helps. If you have any follow-up questions, please let me know. I would be happy to help.