Bug #5: extractAudio invoked ffmpeg without `-map`, so default stream
selection kept only one audio track from sources with multiple audio
streams (e.g. eng/fra/jpn Blu-rays). It now enumerates audio streams
from the streamLangs already fetched by the caller and runs one
`ffmpeg -map 0🅰️<n>` per stream, writing audio.<n>.wav.
Bug #11: the audio language metadata loop in encodeVideo computed the
output index by counting source-side audio streams with a lower Index,
which drifted when some source streams lacked a language tag. It now
walks opusFiles in output order and looks up the language at the
matching source-audio position via a sorted helper.
These ship together because #11 was masked by #5: when only one audio
track survived extraction, the broken index calculation never produced
a visible misalignment. Fixing #5 alone would have caused multi-track
outputs with shuffled language tags; both fixes are required to land
correct multi-track output.
Esa Kataja
022d131cd6