MP3, M4A, FLAC etc) now uses AVFoundation instead of QTKit. As part of the transition from 32-bit to 64-bit technology, Apple has dropped support entirely for QTKit.
Scrubbing and Seeking, including backwards play. 2.1.1: 15 July 2015 Improvements to effects. We've deprecated it now as it does not run on 64 bit. For a long time this was the best Mac build for VI users / VoiceOver, with older versions of macOS (pre-Mojave).