Error on impossible audio conversion

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By default wav => mp3+ogg is generated. With --ffmpeg ffmpeg, anything => mp4+ogg is generated.

Without --ffmpeg, but with specific encoder flags:

--ogg ffmpeg -nostdin -i {in} -b:a 96k -ar 44100 {out}: (wav or ogg or mp3 (lame)) => mp3+ogg

And if you add to --ogg aac: --aac ffmpeg -nostdin -i {in} -b:a 96k -ar 44100 {out}: (wav or ogg or mp3 (lame)) => mp4+ogg