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Logs are recorded every day, stored for a week, then shredded. See logrotate for technical information.
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Administrators (currently only Amolith) can view your IP address, operating system and operating system version, Mumble client version, and public certificate.
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Voice data is encrypted in transport (more information)
Keep in mind that, while Mumble provides recording functionality and there are visual indicators when it's being used, malicious individuals could be recording your conversation with other software. Mumble should be considered as private as IRC (not) and the same precautions taken with both.