DIAGNOSIS You can't fully disable the critical alert voice prompts (like "please install the dirty water tank"), but you can turn off the more frequent instructional and status ones.
FIX Make sure the vacuum is on and in standby (not cleaning). Press and hold the "Clean Water Tank Release" button and the "Power" button simultaneously for about 3 seconds. You'll hear a voice confirmation saying "Voice prompts off" or similar. To turn them back on, repeat the same button combo.
IF THAT DIDN'T WORK Some firmware versions have the setting in the Tineco app. If you have it connected, go to Device Settings within the app and look for a "Voice" or "Audio" toggle. This isn't available on all app versions.
HEADS UP Even with voice "off," it will still speak for critical errors like tank missing, roller jam, or motor fault. You can't silence those, and you probably don't want to—they're the only indicator something is seriously wrong.