The new MC build uses updated Vorbis decoding and encoding source code.
The decoder change is completely internal. The replaced code was rather old and the new code contains some official fixes from xiph.org.
MC13.0.161 or later downloads a new Ogg Vorbis encoder when Ogg Vorbis encoding is accessed for the first time. It is based on the official source, but it is not the famous aoTuV version.
The official source is a safe choice, but the aoTuV version is known to be state of the art Vorbis codec and with some material it produces audibly better quality, especially at low bitrates.
The previous Ogg Vorbis encoder in MC13 was the only aoTuV version that has ever been labeled as "release", namely R1. Since that version the aoTuV developer has released several new beta versions. Despite the "beta" label the aoTuV encoders are stable and commonly known to be safe to use. If you want to continue to use an aoTuV branch encoder you can download the latest exe build for your processor type from
http://www.rarewares.org/ogg-oggenc.php (currently aoTuVb5.7). You can also use the "generic" aoTuV version, but the processor specific versions are a bit faster.
To install an aoTuV encoder, unzip the package you downloaded and place the oggenc2.exe file in the [MC program path]\Plugins\ folder. MC will automatically use the new encoder and will not replace it even when a new MC build is installed.