The audio is yours
Files you generate are yours to use, including in work you are paid for. There is no watermark and no credit is required. The voices come from Common Voice under CC0 1.0, which attaches no conditions.
A courtesy rather than a condition: if a project of yours leans on these voices, a line pointing at Mozilla Common Voice helps the people who recorded them.
What not to put in
Do not use it to impersonate a real person. Building a voice from someone else’s recording without their agreement is not a grey area.
Do not paste anything confidential. Generated files sit on the server for 24 hours and anyone with the exact link can fetch them.
Do not automate it. The limits are there so that the machine stays available to everyone; a script that hammers them takes the site away from other people.
What is not guaranteed
Availability. This runs on one machine, maintained by one person, for free. It can be down.
Accuracy. The transcription is a rule system and the voices are built by machine. Both are measured on this site rather than assumed, and both get things wrong. Check anything that matters.
Permanence. Files are deleted after 24 hours. Download what you want to keep.