No.

Encryption makes it impossible for anyone who is not the sender or recipient of a message to know content of message. All Tox communications (text, audio, video, file transfers, etc) are encrypted. That includes perfect forward secrecy, so all other Tox sessions (including those with the same friend) are safe in the case that someone manages to decrypt one session. This is not negotiable; it is the only mode Tox will operate in.

Thanks to public key cryptography, it is also impossible to pretend to be another Tox user, unless you were to steal that user's private key.

Tox uses the encryption algorithms present in the NaCl crypto library.

“<irungentoo> (…) curve25519 for the key exchange, xsalsa20 for the encryption and poly1305 for the MAC”

More info: https://github.com/irungentoo/toxcore/blob/master/docs/updates/Crypto.md

The encryption used in Tox not only provides same features as OTR, but is also the default, and only operation mode of Tox.

Yes! See Tox over Tor (ToT).

Check out nodes for an updated list.

Opus for audio and VPX (currently 8) for video.

SIP is not being used.

“<irungentoo> toxcore packets are encrypted with MACed crypto so data will never be corrupted in transit”

It basically means that data won't ever be corrupted when sending between Tox clients.

Tox generates a temporary public/private key pair used to make connections to peers in the DHT. Onion routing is used to store and locate Tox IDs, to make it more difficult to, for example, associate Alice and Bob together by who they are looking for in the network.