Use FFmpeg AC-3 encoder rather than Aften A/52

[GammaBoost]

The Aften A/52 encoder is quite out of date and not as good as the encoder bundled with FFmpeg (and used in programs such as Audacity). For example, at 192kbps, FFmpeg has a full 20khz audio spectrum, whereas Aften peaks at around 18khz top. At 128kbps, Aften reaches only about 12khz while FFmpeg reaches 13.5khz.