Authentication failed is a known problem related to special characters in link description.
If you see the following error message:
LTI VoiceThread believes there is an error: The authentication failed; the signature on your request did not match the expected value.
remove the description field in the LTI link or make sure that it does not contain special characters. It is best to have it empty. A video explaining this known problem is linked here.
Other known problems with VoiceThread are at the bottom of the vendor's support page linked here.